v3: preserve map guards in return match lowering

[medvednikov]

What changed

• Preserve map lookup presence checks when return-if chains are created while lowering return-match expressions.
• Bind the typed map value only after map__get_check succeeds.
• Add regression coverage for both the first guard and a chained else-if guard.

Why

The previous return-match lowering transformed a guarded map index into a plain map__get value before C generation. The remaining guard then treated FastcLocal and string values as optional wrappers and emitted invalid .ok and .value field accesses, producing 11 C compiler errors during V3 self-hosting.

Tests

• ./vnew -run-only test_return_match_map_lookup_guard_keeps_presence_check vlib/v3/tests/review_transform_regressions_test.v
• ./vnew -silent test vlib/v3/transform/
• cd vlib/v3 && ../../vnew -gc none -prealloc -o v3 v3.v && ./v3 -nocache -building-v -o v4 v3.v