fixes #26045; #26046; when nimvm leak push options (#26047)

fixes #26045;
fixes #26046

The fix isolates compiler option state while semantically checking each
when nimvm branch.

compiler/semexprs.nim:2745 snapshots the option stack, compiler options,
diagnostics settings, and enabled features. It analyzes one branch and
restores that state in finally. Both the nimvm and else branches use
this function.

This prevents:

```nim
when nimvm:
  {.push overflowChecks: off.}
```

from disabling overflow checks in following runtime code. It also means
a {.pop.} in the opposite branch correctly reports that it has no
corresponding {.push.}.

(cherry picked from commit f6651e6c70)

compiler/semexprs.nim

@@ -2664,6 +2664,22 @@ proc semMagic(c: PContext, n: PNode, s: PSym, flags: TExprFlags; expectedType: P
   else:
     result = semDirectOp(c, n, flags, expectedType)
 
+proc semNimvmBranch(c: PContext, n: PNode, flags: TExprFlags): PNode =
+  let
+    oldOptionStack = c.optionStack[0..^1]
+    oldOptions = c.config.options
+    oldNotes = c.config.notes
+    oldWarningAsErrors = c.config.warningAsErrors
+    oldFeatures = c.features
+  try:
+    result = semExpr(c, n, flags)
+  finally:
+    c.optionStack = oldOptionStack
+    c.config.options = oldOptions
+    c.config.notes = oldNotes
+    c.config.warningAsErrors = oldWarningAsErrors
+    c.features = oldFeatures
+
 proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
   # If semCheck is set to false, ``when`` will return the verbatim AST of
   # the correct branch. Otherwise the AST will be passed through semStmt.
@@ -2700,7 +2716,7 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
       checkSonsLen(it, 2, c.config)
       if whenNimvm:
         if semCheck:
-          it[1] = semExpr(c, it[1], flags)
+          it[1] = semNimvmBranch(c, it[1], flags)
           typ = commonType(c, typ, it[1].typ)
         result = n # when nimvm is not elimited until codegen
       elif c.inGenericContext > 0:
@@ -2731,7 +2747,8 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
         discard
       elif result == nil or whenNimvm:
         if semCheck:
-          it[0] = semExpr(c, it[0], flags)
+          it[0] = if whenNimvm: semNimvmBranch(c, it[0], flags)
+                  else: semExpr(c, it[0], flags)
           typ = commonType(c, typ, it[0].typ)
           if typ != nil and typ.kind != tyUntyped:
             it[0] = fitNode(c, typ, it[0], it[0].info)

tests/whenstmt/twhen_nimvm_push.nim

@@ -0,0 +1,21 @@
+discard """
+  output: "ok"
+"""
+
+var overflowDetected = false
+when nimvm:
+  {.push overflowChecks: off.}
+else:
+  var branchX = high(int)
+  try:
+    inc branchX
+  except OverflowDefect:
+    overflowDetected = true
+
+doAssert overflowDetected
+
+var x = high(int)
+try:
+  inc x
+except OverflowDefect:
+  echo "ok"

tests/whenstmt/twhen_nimvm_push_pop.nim

@@ -0,0 +1,8 @@
+discard """
+  errormsg: "{.pop.} without a corresponding {.push.}"
+  line: 8
+"""
+
+when nimvm:
+  {.push checks: off.}
+else: {.pop.}