[codegen,amdgpu] Enhance MIR DIE and re-arrange it for AMDGPU.

Summary:
- `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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72709