[llvm] [InstSimplify] Forbid undef folds in foldOrCommuteConstant when undef cannot be used. (PR #170670)

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dianqk wrote:

I managed to reduce a bit.

`%i8` can be folded to -1, so `%arg1` can be folded to undef.
First, we try to expand `%i13 = and i8 %i9, %i12` to `%i13 = L1(%arg1(undef) and %i12) or (-1 and %i12)`.
Then, we try to expand `L1(%arg1(undef) and %i12)` to `(%arg1(undef) and %i7) xor (%arg1(undef) and -1)`.
For `%arg1(undef) and %i7`, `%arg1(undef)` is folded to 0, whereas for `%arg1(undef) and -1`, it is folded to undef.


https://github.com/llvm/llvm-project/pull/170670