[llvm] [AMDGPU] Fix GCNUpwardRPTracker: max register pressure on defs. (PR #74422)

[Jay Foad via llvm-commits]

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

LGTM.

The idea here is that all defs (whether they are early-clobber or not) are simultaneously live "at" the instruction, right? Hopefully that matches how the register allocator handles them.

https://github.com/llvm/llvm-project/pull/74422