InstCombine canonicalizes `(select P (and X (- X)) X)` to
`(and (select P (- X) umax) X)`. This is counterproductive for the X86
backend when BMI is available because we can encode `(and X (- X))`
using the `BLSI` instruction. A similar situation arises if we have
`(select P (and X (sub X 1)) X)` (prevents use of `BLSR` instruction) or
`(select P (xor X (sub X 1)) X)` (prevents use of `BLSMSK` instruction).
Trigger the inverse transformation in the X86 backend if BMI is
available and we can use the mentioned BMI instructions. This is done by
overriding the appropriate `shouldFoldSelectWithIdentityConstant()`
overload. In this way, we get `(select P (and X (- X)) X)` again, which
enables the use of `BLSI` (similar for the other cases described above).
Alive proofs: https://alive2.llvm.org/ce/z/MT_pKiFixes#131587, fixes#133848.
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Co-authored-by: Simon Pilgrim <llvm-dev@redking.me.uk>
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