lowering: fix @goto into while loop causing UndefVarError (#61148)
When `@goto` jumps into a while loop body, the initialization of the
`loop_exit_result` variable at the top of the symbolicblock is skipped.
When `break` then jumps to the end of the symbolicblock, the
uninitialized variable is read, causing an UndefVarError.
Fix this by replacing the upfront initialization with an `isdefined`
check at the symbolicblock exit. If the result variable was never
assigned (because the entry was skipped via `@goto`), it falls back to
`nothing`.
Fixes #61129
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Co-authored-by: Keno Fischer <Keno@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>