AMDGPU/PromoteAlloca: Refactor into analysis / commit phases

This change is motivated by the overall goal of finding alternative ways
to promote allocas to VGPRs. The current solution is effectively limited
to allocas whose size matches a register class, and we can't keep adding
more register classes. We have some downstream work in this direction,
and I'm currently looking at cleaning that up to bring it upstream.

This refactor paves the way to adding a third way of promoting allocas,
on top of the existing alloca-to-vector and alloca-to-LDS. Much of the
analysis can be shared between the different promotion techniques.

Additionally, the idea behind splitting the pass into an analysis
phase and a commit phase is that it ought to allow us to more easily make
better "big picture" decision about which allocas to promote how in the
future.

commit-id:138f5985

Author: nhaehnle