Sanjay Patel ca7eaa0a54 [InstSimplify] allow undef element match in vector select condition value ...

The semantics of select with undefined/poison condition
are not explicitly stated in the LangRef, but this matches
comments in the code and Alive2 appears to concur:
https://alive2.llvm.org/ce/z/KXytmd

We can find this pattern after demanded elements transforms.

As noted in D101191, fuzzers are finding infinite loops because
we may not account for this pattern in other passes.

2021-05-25 14:25:34 -04:00

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