fixes branches interacting with break, raise etc. in strictdefs (#22627)

```nim
{.experimental: "strictdefs".}

type Test = object
  id: int

proc test(): Test =
  if true:
    return Test()
  else:
    return
echo test()
```

I will tackle https://github.com/nim-lang/Nim/issues/16735 and #21615 in
the following PR.


The old code just premises that in branches ended with returns, raise
statements etc. , all variables including the result variable are
initialized for that branch. It's true for noreturn statements. But it
is false for the result variable in a branch tailing with a return
statement, in which the result variable is not initialized. The solution
is not perfect for usages below branch statements with the result
variable uninitialized, but it should suffice for now, which gives a
proper warning.

It also fixes

```nim

{.experimental: "strictdefs".}

type Test = object
  id: int

proc foo {.noreturn.} = discard

proc test9(x: bool): Test =
  if x:
    foo()
  else:
    foo()
```
which gives a warning, but shouldn't

2 files changed, 157 insertions, 0 deletions

diff --git a/tests/init/tcompiles.nim b/tests/init/tcompiles.nim
new file mode 100644
index 000000000..2072702ad
--- /dev/null
+++ b/tests/init/tcompiles.nim
@@ -0,0 +1,64 @@
+discard """
+  matrix: "--warningAsError:ProveInit --warningAsError:Uninit"
+"""
+
+{.experimental: "strictdefs".}
+
+type Test = object
+  id: int
+
+proc foo {.noreturn.} = discard
+
+block:
+  proc test(x: bool): Test =
+    if x:
+      foo()
+    else:
+      foo()
+
+block:
+  proc test(x: bool): Test =
+    if x:
+      result = Test()
+    else:
+      foo()
+
+  discard test(true)
+
+block:
+  proc test(x: bool): Test =
+    if x:
+      result = Test()
+    else:
+      return Test()
+
+  discard test(true)
+
+block:
+  proc test(x: bool): Test =
+    if x:
+      return Test()
+    else:
+      return Test()
+
+  discard test(true)
+
+block:
+  proc test(x: bool): Test =
+    if x:
+      result = Test()
+    else:
+      result = Test()
+      return
+
+  discard test(true)
+
+block:
+  proc test(x: bool): Test =
+    if x:
+      result = Test()
+      return
+    else:
+      raise newException(ValueError, "unreachable")
+
+  discard test(true)
diff --git a/tests/init/treturns.nim b/tests/init/treturns.nim
new file mode 100644
index 000000000..77469472a
--- /dev/null
+++ b/tests/init/treturns.nim
@@ -0,0 +1,93 @@
+{.experimental: "strictdefs".}
+
+type Test = object
+  id: int
+
+proc foo {.noreturn.} = discard
+
+proc test1(): Test =
+  if true: #[tt.Warning
+  ^ Cannot prove that 'result' is initialized. This will become a compile time error in the future. [ProveInit]]#
+    return Test()
+  else:
+    return
+
+proc test0(): Test =
+  if true: #[tt.Warning
+  ^ Cannot prove that 'result' is initialized. This will become a compile time error in the future. [ProveInit]]#
+    return
+  else:
+    foo()
+
+proc test2(): Test =
+  if true: #[tt.Warning
+  ^ Cannot prove that 'result' is initialized. This will become a compile time error in the future. [ProveInit]]#
+    return
+  else:
+    return
+
+proc test3(): Test =
+  if true: #[tt.Warning
+  ^ Cannot prove that 'result' is initialized. This will become a compile time error in the future. [ProveInit]]#
+    return
+  else:
+    return Test()
+
+proc test4(): Test =
+  if true: #[tt.Warning
+  ^ Cannot prove that 'result' is initialized. This will become a compile time error in the future. [ProveInit]]#
+    return
+  else:
+    result = Test()
+    return
+
+proc test5(x: bool): Test =
+  case x: #[tt.Warning
+  ^ Cannot prove that 'result' is initialized. This will become a compile time error in the future. [ProveInit]]#
+  of true:
+    return
+  else:
+    return Test()
+
+proc test6(x: bool): Test =
+  case x: #[tt.Warning
+  ^ Cannot prove that 'result' is initialized. This will become a compile time error in the future. [ProveInit]]#
+  of true:
+    return
+  else:
+    return
+
+proc test7(x: bool): Test =
+  case x: #[tt.Warning
+  ^ Cannot prove that 'result' is initialized. This will become a compile time error in the future. [ProveInit]]#
+  of true:
+    return
+  else:
+    discard
+
+proc test8(x: bool): Test =
+  case x: #[tt.Warning
+  ^ Cannot prove that 'result' is initialized. This will become a compile time error in the future. [ProveInit]]#
+  of true:
+    discard
+  else:
+    raise
+
+proc hasImportStmt(): bool =
+  if false: #[tt.Warning
+  ^ Cannot prove that 'result' is initialized. This will become a compile time error in the future. [ProveInit]]#
+    return true
+  else:
+    discard
+
+discard hasImportStmt()
+
+block:
+  proc hasImportStmt(): bool =
+    if false: #[tt.Warning
+    ^ Cannot prove that 'result' is initialized. This will become a compile time error in the future. [ProveInit]]#
+      return true
+    else:
+      return
+
+  discard hasImportStmt()