[AArch64] Optimize CBZ wzr and friends.

[davemgreen]

In certain situations, especially with zero phi operands propagated after tail duplications, we can end up with CBZ/CBNZ/TBZ/TBNZ with a zero register. It only happens late in the pipeline. This patch adds a basic simplifyInstruction to fold them away to either a direct branch or removing the instruction entirely.

It needs some fixups, but seems to work.

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff origin/main HEAD --extensions h,cpp -- llvm/include/llvm/CodeGen/TargetInstrInfo.h llvm/lib/CodeGen/MachineCopyPropagation.cpp llvm/lib/CodeGen/ShrinkWrap.cpp llvm/lib/Target/AArch64/AArch64InstrInfo.cpp llvm/lib/Target/AArch64/AArch64InstrInfo.h llvm/lib/Target/RISCV/RISCVInstrInfo.cpp llvm/lib/Target/RISCV/RISCVInstrInfo.h

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View the diff from clang-format here.

diff --git a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
index 99a182012..4cdd370e2 100644
--- a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
@@ -704,7 +704,7 @@ bool AArch64InstrInfo::simplifyInstruction(MachineInstr &MI,
       for (auto *S : Succs)
         if (S != Target)
           MBB->removeSuccessor(S);
-      SmallVector<MachineInstr*> DeadInstrs;
+      SmallVector<MachineInstr *> DeadInstrs;
       for (auto It = MI.getIterator(); It != MBB->end(); ++It)
         DeadInstrs.push_back(&*It);
       BuildMI(MBB, MI.getDebugLoc(), get(AArch64::B)).addMBB(Target);

[dtellenbach]

Would it make sense to also check for cb.{eq,ne} {x,w}zr, #0? Not sure if we'd ever codegen that though.