[llvm] [AArch64] Add patterns for selecting dec from sub (PR #209467)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

To me this feels like it should be a canonicalisation to remove the need for what's effectively a duplicate set of patterns.  Looking at `DAGCombiner::visitSUB()` I can see the combine exists but is currently restricted to single uses.  I don't see a reason for that and think "vscale(imm)" should be treated like any other constant.

https://github.com/llvm/llvm-project/pull/209467