[LateLowerGC] Change undef FCA into zeroinit #43867
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vchuravy
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vtjnash
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Fixes #43798
@vtjnash and I discussed this offline. The long-term proper fix maybe to not from FCA entirely in the front-end.
For now we settled on the fact that we can scan the IR and make sure that formed FCA are not undef initialized,
but instead zero initialized, ensuring that LateLowerGC didn't synthesize a use of an undef value.
An alternative proposal by me was to use
ValueTrackingto see if something is proven undef, but that seemed rather costlyand more invasive.