[AMDGPU] Use SSAUpdater in PromoteAlloca

This allows PromoteAlloca to not be reliant on a second SROA run to remove the alloca completely. It just does the full transformation directly.

Note PromoteAlloca is still reliant on SROA running first to
canonicalize the IR. For instance, PromoteAlloca will no longer handle aggregate types because those should be simplified by SROA before reaching the pass.

Reviewed By: #amdgpu, arsenm

Differential Revision: https://reviews.llvm.org/D152706