[AMDGPU] Add Optimize VGPR LiveRange Pass.
This pass aims to optimize VGPR live-range in a typical divergent if-else
control flow. For example:
use(a) ... // 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
The pass is enabled by default, the frontend can disable it through
Differential Revision: https://reviews.llvm.org/D102212