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

[via llvm-commits]

easyonaadit wrote:

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

AMDGPU has a growing-up stack, all calculations are based on that.

The calculations are based on if there has been a dynamic allocation. 
The prolog does: `sp += frameSize + alignment`
and the epilog un-does this.
If there has been a dynamic allocation, then sp is not at the location the epilog expects it at, so the calculations give incorrect restoration of sp.
This patch is just returning sp to the location at which the epilog expects it.

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