[PowerPC] Adjust MaxAtomicSizeInBitsSupported on PPC64

AtomicExpandPass uses this variable to determine emitting libcalls or not. The default value is 1024 and if we don't specify it for PPC64 explicitly, AtomicExpandPass won't emit `__atomic_*` libcalls for those target unable to inline atomic ops and finally the backend emits `__sync_*` libcalls. Thanks @efriedma for pointing it out.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D122868

Author: Kai Luo

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

@@ -1322,7 +1322,6 @@ PPCTargetLowering::PPCTargetLowering(const PPCTargetMachine &TM,
   }
 
   if (shouldInlineQuadwordAtomics()) {
-    setMaxAtomicSizeInBitsSupported(128);
     setOperationAction(ISD::ATOMIC_LOAD, MVT::i128, Custom);
     setOperationAction(ISD::ATOMIC_STORE, MVT::i128, Custom);
     setOperationAction(ISD::INTRINSIC_VOID, MVT::i128, Custom);
@@ -1347,6 +1346,10 @@ PPCTargetLowering::PPCTargetLowering(const PPCTargetMachine &TM,
 
   if (!isPPC64)
     setMaxAtomicSizeInBitsSupported(32);
+  else if (shouldInlineQuadwordAtomics())
+    setMaxAtomicSizeInBitsSupported(128);
+  else
+    setMaxAtomicSizeInBitsSupported(64);
 
   setStackPointerRegisterToSaveRestore(isPPC64 ? PPC::X1 : PPC::R1);