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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

As usual when touching SimplifyDemandedBits et al this has managed to expose a bug somewhere else that only shows up in multi stage build bots - currently looking at this.

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