[llvm] [AArch64][GlobalISel] Add a constant funnel shift post-legalizer combine. (PR #151912)

[Amara Emerson via llvm-commits]

aemerson wrote:

That would be a nice capability to have. We should probably have this discussion on an RFC, but my main concern right now is that we'd need to be resilient if the constant was obscured somehow; and that the resiliency would add complexity that nets us little benefit in the end.

There's maybe some other alternatives, like having specialized opcodes that encode the constant as an operand instead of a reg.

https://github.com/llvm/llvm-project/pull/151912