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

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

I don't fully understand the RISCV changes but when investigating vwadd_vx_splat_sext_i1 it looks like a target specific DAG combine is now firing for `SUB x, SPLAT(imm)` which was previously blocked because a freeze sat between the SUB and SPLAT.

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