[llvm] [AArch64] recognise zip1/zip2 with flipped operands (PR #167235)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

>> I always though we were or should be canonicalising shuffles so that the first index is for the LHS.
> It is definitely possible to make clang -O2 -emit-llvm generate code where that is not the case (https://godbolt.org/z/Yx4TToebb):

Yeah, it depends on the shuffle but many are not. I was wondering if we should - to avoid the need to try and match every pattern in multiple ways. It would fix them all at once, (and we could make the perfect shuffle tables smaller). There are DAG canonicalizations that get in the way though, for shuffle(x, shuffle), so this is probably OK.

https://github.com/llvm/llvm-project/pull/167235