[llvm] [AMDGCN][SIWholeQuadMode] Handle case when SI_KILL_I1_TERMINATOR -1,0 is not the only terminator (PR #122922)

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

The `splitBlock` changes look fine and aren't strictly related to the rest of the patch, right? Can you commit them separately?

https://github.com/llvm/llvm-project/pull/122922