[AMDGPU] Add Optimize VGPR LiveRange Pass.

This pass aims to optimize VGPR live-range in a typical divergent if-else
control flow. For example:

def(a)
if(cond)
  use(a)
  ... // A
else
  use(a)

As AMDGPU access vgpr with respect to active-mask, we can mark `a` as
dead in region A. For details, please refer to the comments in
implementation file.

The pass is enabled by default, the frontend can disable it through
"-amdgpu-opt-vgpr-liverange=false".

Differential Revision: https://reviews.llvm.org/D102212