[llvm] AMDGPU: Use LiveIntervals instead of LiveVariables in SIOptimizeVGPRLiveRange (PR #227252)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> We see the same failure in (at least) one of our buildbots: https://lab.llvm.org/buildbot/#/builders/10/builds/36734

That was just a midair collision fixed in #227621

https://github.com/llvm/llvm-project/pull/227252