[llvm] [AArch64] Avoid applying S-form on frame index in peephole (PR #158597)

[via llvm-commits]

AZero13 wrote:

> Question: would it make more sense for this to be in the .td files if possible? Or?

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

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