[llvm] [X86] combinePCLMULQDQ - attempt to fold PCLMULQDQ(SHUFFLE(X),SHUFFLE(Y),C) -> PCLMULQDQ(X,Y,C') (PR #176932)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Reduced:
```ll
define <2 x i64> @pclmul_sentinel_zero(<2 x i64> %a0, <2 x i64> %a1) {
  %and = and <2 x i64> %a0, <i64 -1, i64 0>
  %res = tail call <2 x i64> @llvm.x86.pclmulqdq(<2 x i64> %and, <2 x i64> %a1, i8 0)
  ret <2 x i64> %res
}
```

https://github.com/llvm/llvm-project/pull/176932