[llvm] [AArch64] NFC: rename subreg zsub to qsub (PR #111154)

[via llvm-commits]

zmodem wrote:

We're hitting an assert after this change:

```
Assertion failed: (Subtarget.isSVEorStreamingSVEAvailable() && "Unexpected register store without SVE store instructions"), function storeRegToStackSlot, file AArch64InstrInfo.cpp, line 5346.
```

See https://crbug.com/372053411#comment3 for a reproducer.

I'll prepare a revert until this can be fixed.

https://github.com/llvm/llvm-project/pull/111154