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

[Jay Foad via llvm-commits]

jayfoad wrote:

Let me try again. Before your patch si-wqm generates:
```
256B	bb.2.endif1:
	; predecessors: %bb.0, %bb.1
	  successors: %bb.4(0x80000000); %bb.4(100.00%)

272B	  $exec = S_OR_B64 $exec, %0:sreg_64, implicit-def $scc
288B	  S_BRANCH %bb.3
292B	  S_BRANCH %bb.4
```
Your patch removes the _first_ of these branches. How can that be correct? The _second_ branch is dead code, so removing that would make a lot more sense.

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