[llvm] [GlobalISel] Use G_ANYEXT instead of G_ZEXT for shift amount widening (PR #223136)

[David Green via llvm-commits]

davemgreen wrote:

Hi - See https://github.com/llvm/llvm-project/pull/199870#pullrequestreview-4374220494 and https://github.com/llvm/llvm-project/pull/213576#issuecomment-5176469875 for where this came up before. @YeonguChoe FYI.

I think this needs to be performed in the selector, similar to why DAG does it in AArch64DAGToDAGISel::tryShiftAmountMod.

https://github.com/llvm/llvm-project/pull/223136