[llvm] [AArch64] Unfold adds when eliminating frame index with scalable offset (PR #158597)

[Hongyu Chen via llvm-commits]

XChy wrote:

> Basically, the *.td makes all subs the s form by default. Should it make it not do that for frame index?

If that's the case, we should unfold it in PEI.

https://github.com/llvm/llvm-project/pull/158597