[llvm] [AArch64] Fold COPY(y:gpr, DUP(x:fpr, i)) -> UMOV(y:gpr, x:fpr, i) (PR #89017)

[Dhruv Chawla via llvm-commits]

dc03-work wrote:

> AArch64MIPeepholeOpt was added as a last-resort for things we didn't know where else to put. In the long run I feel we really need reg-bank select to be doing it's job properly even if it means selecting regbanks more lazily.
> 
> AArch64PostSelectOptimize sounds like a decent compromise. Could it be made to fix the regressions you see at the same time?

@davemgreen I was able to do this. Please have a look.

https://github.com/llvm/llvm-project/pull/89017