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

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

> You probably want to add handling for SPLAT_VECTOR in getValidShiftAmountRange

I've taken a look and think getValidShiftAmountRange already works?  It'll drop into `computeKnownBits` which already handles SPLAT_VECTOR.

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