8369979: Flag UsePopCountInstruction was accidentally disabled on PPC64

Backport-of: 9b9559a2e33827126e1aeab7bf6f4861acaae109

Author: dbriemann

src/hotspot/cpu/ppc/vm_version_ppc.cpp

@@ -97,6 +97,10 @@ void VM_Version::initialize() {
     FLAG_SET_ERGO(TrapBasedRangeChecks, false);
   }
 
+  if (FLAG_IS_DEFAULT(UsePopCountInstruction)) {
+    FLAG_SET_ERGO(UsePopCountInstruction, true);
+  }
+
   if (PowerArchitecturePPC64 >= 9) {
     // Performance is good since Power9.
     if (FLAG_IS_DEFAULT(SuperwordUseVSX)) {