[AArch64] Use isAllOnes rather than popcount() == Size (NFC) (#167884)

Author: AZero13

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -2558,7 +2558,7 @@ bool AArch64TargetLowering::targetShrinkDemandedConstant(
     return false;
 
   // Exit early if we demand all bits.
-  if (DemandedBits.popcount() == Size)
+  if (DemandedBits.isAllOnes())
     return false;
 
   unsigned NewOpc;