[llvm] [AMDGPU] Restore SP from saved-FP or saved-BP (PR #124007)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > (Note: The presence of BP has been enforced in case of any
> > dynamic stack realignment.)
> 
> Another optimization opportunity: BP is only really needed if you have _both_ dynamic stack realignment and variable-sized objects. Otherwise there is at most one alignment gap and you can use FP/SP to access objects before/after the gap.

Can you do this in a follow up? 

https://github.com/llvm/llvm-project/pull/124007