[llvm] [AMDGPU] Make sort ordering in `AMDGPURewriteAGPRCopyMFMAImpl::eliminateSpillsOfReassignedVGPRs()` strict weak. (PR #162493)

[weiwei chen via llvm-commits]

weiweichen wrote:

Merge for now. Will add a test if @arsenm has suggestions on how to add test for this (sorry, I need some help with adding AMDGPU unit test)

https://github.com/llvm/llvm-project/pull/162493