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

[Dhruva Chakrabarti via llvm-commits]

dhruvachak wrote:

> I'm a confused about why #200126 is only additive, and why this isn't more directly computed from the LiveIntervals/LiveStacks. You shouldn't really need to research through blocks for the frame index references? If #200126 is finding where to put the IMPLICIT_DEF, then why do we need all of this code to skip this scenario?

If you are asking why I split up the work into 2 PRs, the reason is mostly about users waiting for this crash to be fixed. I agree that PR#1 bails out while PR#2 tries to proceed with the unspill, so that way they are not completely aligned. But PR#2 builds on top of the first, essentially taking off where the first left off. 

I felt that more discussion would be required for PR#2 to converge but we could perhaps agree on PR#1 faster.

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