[llvm] [DAG] Replace getValid*ShiftAmountConstant helpers with getValid*ShiftAmount helpers to support KnownBits analysis (PR #93182)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Thanks everyone, unless anyone has any more concerns I'm intending to commit this over the weekend.

https://github.com/llvm/llvm-project/pull/93182