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

[Carl Ritson via llvm-commits]

perlfu wrote:

> 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`???

I had a look, but I will could not be sure that either of these would work.
`handleMove` certainly doesn't at present -- partly because the instruction itself does not necessarily move slot, only the def it creates.
That said, I do wonder if `handleMove` could/should be extended to cover this case.

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