[llvm] [BOLT][AArch64] Do not crash on authenticated branch instructions (PR #129898)

[Anatoly Trosinenko via llvm-commits]

atrosinenko wrote:

> Current implementation for aarch64 supports only BR as indirect instruction for Jump tables, but I am working on the jump tables functionality right now, and I see a lot of JT with BLR instruction. If you are going to use BLRAA or other instructions it's worth to add checking for them as well

Now got it, thank you! I thought these were the same four `AArch64::BRA*` instructions, but these were their "with link" counterparts. Could you please provide an example of such JT lowering - using "branch with link" looks quite counter-intuitive at first.

> as example I think we can create the different function isIndirectAuthBranch or we can use if (isIndirectBranchOpcode() & !Inst.getOpcode() != AArch64::BR)
>
> If we can avoid duplication then let try to do it, if not you can leave it as is

Testing for `isIndirectBranchOpcode() & !Inst.getOpcode() != AArch64::BR` looks like replacing
```cpp
    assert(Inst.getOpcode() == AArch64::BR && "Unexpected opcode");
```
with
```cpp
    if (Inst.getOpcode() != AArch64::BR)
      return;
```
This could make sense as "there are many variants of `Inst.getOpcode()`, and we only handle this specific one", but an assertion looks reasonable as it somewhat documents "we handle this opcode, we know about these opcodes and intentionally ignore them and we do not expect anything else here". Considering defining yet another `isIndirectAuthBranch` function - I would prefer to stick to the existing approach of defining BOLT-specific `isXYZ(Opcode)`  for this PR (I briefly grepped the existing existing sources and did not find other possible users of this function so far), but I spotted that multiple styles for `isXYZ(Opcode)` functions are already used in `AArch64MCPlusBuilder.cpp`, so I changed `isBRA` to use more concise switch-case style.

https://github.com/llvm/llvm-project/pull/129898