Fix return status of AtomicExpandPass

[serge-sans-paille]

Summary

Correctly reflect change in the return status.

Patch needed to land https://reviews.llvm.org/D80916

[foad]

Looks obviously correct. Might be slightly neater to change expandPartwordCmpXchg to return true, like expandAtomicCmpXchg does?

llvm/lib/CodeGen/AtomicExpandPass.cpp

 bool AtomicExpand::tryExpandAtomicCmpXchg(AtomicCmpXchgInst *CI) {
   unsigned MinCASSize = TLI->getMinCmpXchgSizeInBits() / 8;
   unsigned ValueSize = getAtomicOpSize(CI);
 
   switch (TLI->shouldExpandAtomicCmpXchgInIR(CI)) {
   default:
     llvm_unreachable("Unhandled case in tryExpandAtomicCmpXchg");
   case TargetLoweringBase::AtomicExpansionKind::None:
-    if (ValueSize < MinCASSize)
+    if (ValueSize < MinCASSize) {
       expandPartwordCmpXchg(CI);
+      return true;
+    }
     return false;
   case TargetLoweringBase::AtomicExpansionKind::LLSC: {
     return expandAtomicCmpXchg(CI);
   }
   case TargetLoweringBase::AtomicExpansionKind::MaskedIntrinsic:
     expandAtomicCmpXchgToMaskedIntrinsic(CI);
     return true;
   }