[llvm] Revert "[AArch64] Define high bits of FPR and GPR registers (take 2) (#114827)" (PR #117307)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Strictly speaking not all patches need reverting (like the changes to InitUndef or RegisterCoalescer), although without the AArch64 changes that would mean those changes would no longer have any tests. I'll reland all these after I've investigated the issue caused by this patch.
> 

This is why it's better to split these into independent patches 


https://github.com/llvm/llvm-project/pull/117307