[llvm] [CodeGen][AMDGPU] Insert IMPLICIT_DEF for undef subreg operands in REG_SEQUENCE and preserve them during register coalescing (PR #189153)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

This is multiple changes, some of which shouldn't be necessary. The crash is fully isolated to running register-coaslecer - the fix should be contained there.

https://github.com/llvm/llvm-project/pull/189153