Partially-undef constants cause a large size regression in src/test/run-make/wasm-panic-small

[erikdesjardins]

Opening this to track a limitation of #83698, which does not generate partially-undef constants (controlled by -Zpartially-uninit-const-threshold=1234) due to this issue.

The root cause is that LLVM is unable to const prop (?) a load from a partially-undef constant, which prevents all the panic formatting code from being removed in the src/test/run-make/wasm-panic-small test, which causes the test to fail.

Minimal repro (from #83698 (comment)): https://godbolt.org/z/3vYqM5865

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per nikic:

https://godbolt.org/z/1Mff6zrvd shows one of the ways this could be fixed on the LLVM side. We should be dropping trailing zeros from bitcasted GEPs.

[erikdesjardins]

Underlying issue should be fixed in the next LLVM release by https://reviews.llvm.org/D111023. Thanks @nikic!

When LLVM 14 is out, I will revisit enabling this.