[AMDGPU] Run unreachable-mbb-elimination after isel to clean up PHIs.
- 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
Differential Revision: https://reviews.llvm.org/D64353