[llvm] [SDAG] fix miss opt: shl nuw + zext adds unnecessary masking (PR #172046)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Have you looked at doing this in SelectionDAGBuilder with the other AssertZExt/AssertSExt handling? Ideally this needs to be attached to the corresponding CopyFromRegs node.

https://github.com/llvm/llvm-project/pull/172046