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.}.
When statement branches exit early outside of nimvm. In nimvm it seems that all
sides of the branches must be evaluated as the code gen happens at a later
stage, this remains intact.
