[codegen,amdgpu] Enhance MIR DIE and re-arrange it for AMDGPU.
- dead-mi-elimination assumes MIR in the SSA form and cannot be arranged after phi elimination or DeSSA. It's enhanced to handle the dead register definition by skipping use check on it. Once a register def is dead, all its uses, if any, should be undef.
- Re-arrange the DIE in RA phase for AMDGPU by placing it directly after detect-dead-lanes.
- Many relevant tests are refined due to different register assignment.
Reviewers: rampitec, qcolombet, sunfish
Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D72709