[llvm] [AArch64] Drop poison-generating flags in `genSubAdd2SubSub` combiner (PR #90028)

[Momchil Velikov via llvm-commits]

momchil-velikov wrote:

It looks like there's a slight issue with the test. If you have enabled expensive checks, the combiner calls `verifyPatternOrder` which creates(and discards) a few instructions, but in doing so consumes some virtual register numbers. Then you get unexpected output in the test, for example, I get `%9` instead of `%7` in these check lines:
```
# CHECK-LABEL: name: test8
# CHECK:       %7:gpr64 = SUBXrr %1, %0
# CHECK-NEXT:  %4:gpr64common = SUBXrr killed %7, killed %2
```

https://github.com/llvm/llvm-project/pull/90028