[InstCombine] Bubble splices of binop operands to their result (#179432)
In #172961 we are trying to remove llvm.experimental.vp.reverse now that
llvm.vector.splice.right supports variable offsets.
A VP reverse reverses the first EVL elements of the vector, e.g.
01234567 -> 210xxxxx when EVL=3, where x=poison.
This can now be represented by splice.right(reverse(V), poison, EVL):
01234567
-> 76543210 (reverse)
-> 210xxxxx (splice.right)
This PR implements the vp.reverse combines that pull through binops, but
generalized to vector.splice. Specifically, this implements the
following combines:
Op(splice(V1, poison, offset), splice(V2, poison, offset)) -> splice(Op(V1, V2), poison, offset)
Op(splice(V1, poison, offset), RHSSplat) -> splice(Op(V1, RHSSplat), poison, offset)
Op(LHSSplat, splice(V2, poison, offset)) -> splice(Op(LHSSplat, V2), poison, offset)
We can then remove the vp.reverse intrinsic and its related combines
soon after, once we migrate the loop vectorizer over.
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