[llvm] [1/2][AMDGPU] Fixed crash due to virtual register defs not dominating uses (PR #198472)

[Dhruva Chakrabarti via llvm-commits]

dhruvachak wrote:

At this point, the technique to place an IMPLICIT_DEF is the subject of another PR https://github.com/llvm/llvm-project/pull/200126. So, I am moving the relevant discussion to that PR. Please let's continue that discussion on that PR. 

https://github.com/llvm/llvm-project/pull/198472