[llvm] [RISCV][ISel] Add codegen support for the experimental zabha extension (PR #80192)

[James Y Knight via llvm-commits]

================
@@ -19512,8 +19515,13 @@ RISCVTargetLowering::shouldExpandAtomicRMWInIR(AtomicRMWInst *AI) const {
     return AtomicExpansionKind::None;
 
   unsigned Size = AI->getType()->getPrimitiveSizeInBits();
-  if (Size == 8 || Size == 16)
-    return AtomicExpansionKind::MaskedIntrinsic;
+  if (Size == 8 || Size == 16) {
+    if (!Subtarget.hasStdExtZabha())
+      return AtomicExpansionKind::MaskedIntrinsic;
+    if (AI->getOperation() == AtomicRMWInst::Nand)
----------------
jyknight wrote:

I find the Nand conditions confusing as written.

How about the following:
```
  [...]
  unsigned Size = AI->getType()->getPrimitiveSizeInBits();
  if (AI->getOperation() == AtomicRMWInst::Nand && Subtarget.hasStdExtZacas() &&
      (Size >= 32 || Subtarget.hasStdExtZabha()))
    return AtomicExpansionKind::CmpXChg;
  if (Size < 32 && !Subtarget.hasStdExtZabha())
    return AtomicExpansionKind::MaskedIntrinsic;
  return AtomicExpansionKind::None;
```

https://github.com/llvm/llvm-project/pull/80192