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

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

> Prolog:
`tmp = sp + (alignment - 1)`
`fp &= -alignment`
`bp = sp`
`sp += frameSize + alignment`
Epilog:
`sp += bp + frameSize + alignment`
`sp -= (frameSize + alignment)`

The epilogue sequence is effectively `sp += bp`, is this correct?
The prologue sequence is also confusing. Usually, `fp` and `bp` have swapped meaning.


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