[AMDGPU] Run `unreachable-mbb-elimination` after isel to clean up PHIs.

Summary:
- As LCSSA is turned on just before isel, it may create PHI of the flow,
  which is consumed by pseudo structurized CFG instructions. When that
  PHIs are eliminated in O0, COPY may be placed wrongly as the these
  pseudo structurized CFG instructions are considering prologue of MBB.
- Run extra `unreachable-mbb-elimination` at the end of isel to clean up
  PHIs.

Reviewers: arsenm, rampitec

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64353

llvm-svn: 367023