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

[David Green via llvm-commits]

davemgreen wrote:

This adds a change to the existing matchOrShiftToFunnelShift so that AArch64 can generate such instructions post-legalization providing that the operation is scalar and the shift amount is constant. It doesn't feel like the best solution - any thoughts on alternatives?

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