PowerPC: Avoid MCSymbol::isUnset

"unset" is not a meaningful separae state and it will be merged into
"undefined".

https://reviews.llvm.org/D72570 ported a GAS hack that moves the label
when a prefixed instruction on the same source line needs automatic
alignment. However, we can used isDefined instead of isUnset.

Author: MaskRay

llvm/lib/Target/PowerPC/MCTargetDesc/PPCELFStreamer.cpp

@@ -65,7 +65,7 @@ void PPCELFStreamer::emitPrefixedInstruction(const MCInst &Inst,
   MCFragment *InstructionFragment = getCurrentFragment();
   SMLoc InstLoc = Inst.getLoc();
   // Check if there was a last label emitted.
-  if (LastLabel && !LastLabel->isUnset() && LastLabelLoc.isValid() &&
+  if (LastLabel && LastLabel->isDefined() && LastLabelLoc.isValid() &&
       InstLoc.isValid()) {
     const SourceMgr *SourceManager = getContext().getSourceManager();
     unsigned InstLine = SourceManager->FindLineNumber(InstLoc);