[llvm] [AArch64][FEAT_CMPBR] Codegen for Armv9.6-a CBB and CBH (PR #164899)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

The case where it goes wrong is something like:

- We MachineCombine away a "no-op" extension instruction
- We do IfConversion to lower a CBB, and the expansion depends on the extension instruction we eliminated.

Obviously, we just need to make sure MachineCombine doesn't do that; it either needs to strip the extension assertion, or skip the optimization.  But I want the opcode name to indicate we have an assertion stapled to the underlying CBB, so someone implementing that MachineCombine knows to look for it.

https://github.com/llvm/llvm-project/pull/164899