[llvm] [LLVM][SelectionDAG] Remove scalable vector restriction from poison analysis. (PR #102504)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM.  I've audited the code, and don't spot any obvious problems, and the general approach is good.  

You probably want to add handling for SPLAT_VECTOR in getValidShiftAmountRange

https://github.com/llvm/llvm-project/pull/102504