[llvm] [AMDGPU] Update LiveInterval def index for early-clobber (PR #79285)

[Jay Foad via llvm-commits]

https://github.com/jayfoad approved this pull request.

Looks fine to me, though I do wonder if it would be possible to use some helper instead of writing lots of new code. Maybe `CalcLiveRangeUtil::extendSegmentStartTo` or `LiveIntervals::handleMove`???

https://github.com/llvm/llvm-project/pull/79285