[llvm] [AMDGPU] Restore SP correctly in functions with dynamic allocas (PR #122743)

[Christudasan Devadasan via llvm-commits]

cdevadas wrote:

@easyonaadit can you see if we could restore the Stack Pointer (SP) in the function epilog directly from FP/BP (based on which frame register holds the stack base at its entry). If that can be done, we can avoid this patch and any other subtract we currently do in the epilog to restore the SP at return as given here:
https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/AMDGPU/SIFrameLowering.cpp#L1284 

https://github.com/llvm/llvm-project/pull/122743