[llvm] [PowerPC] enable AtomicExpandImpl::expandAtomicCmpXchg for powerpc (PR #142395)

[Matt Arsenault via llvm-commits]

================
@@ -1475,6 +1475,10 @@ PPCTargetLowering::PPCTargetLowering(const PPCTargetMachine &TM,
   setMinimumJumpTableEntries(PPCMinimumJumpTableEntries);
 
   setMinFunctionAlignment(Align(4));
+  if (Subtarget.hasPartwordAtomics())
+    setMinCmpXchgSizeInBits(8);
+  else
+    setMinCmpXchgSizeInBits(32);
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arsenm wrote:

```suggestion
    setMinCmpXchgSizeInBits(Subtarget.hasPartwordAtomics() ? 8 : 32);
```

https://github.com/llvm/llvm-project/pull/142395