Fix br_on_cast parsing with non-nullable extra values

[pttydou]

Fixes #8949.

Text parsing did not normalize synthesized non-defaultable scratch locals after finishing each function, unlike binary reading. This applies the existing fixup and preserves exact reference types when building nullable-output br_on_cast fallthrough values.

Tests:

• python out/bin/binaryen-lit -vv test/lit/basic/br-on-cast-extra-values.wast test/lit/basic/scratch-local-roundtrip.wast test/lit/basic/extra-branch-values.wast test/lit/basic/unreachable-br-on-cast-desc.wast test/lit/basic/exact-references.wast test/lit/cast-to-basic.wast
• ruff check

[tlively]

Thanks! LGTM with the test updated.

[tlively]

Please update the test to include the expected output. After updating the RUN line, you can use scripts/update_lit_checks.py to generate the expectations.

Suggested change

	;; RUN: wasm-opt %s -all -S -o %t
	;; RUN: wasm-opt %s -all --roundtrip -S -o %t.roundtrip
	;; RUN: wasm-opt %s -all --roundtrip -S -o - | filecheck %s

[pttydou]

Updated the test to pipe the roundtrip output through FileCheck and generated the expected output with scripts/update_lit_checks.py. The focused lit tests pass locally.

[tlively]

Let's not worry about the firstScratchLocal optimization and just call handleNonDefaultableLocals as-is. It's ok if it fixes locals that were used invalidly in the input; there are a few similar cases where we end up parsing and accepting invalid modules.