PPC: Don't replace weighted branches with ISEL on some targets.

[iteratee]

Summary

On POWER7 and POWER8 an ISEL is comparatively slow compared to a
weighted branch. Don't replace a hinted branch with an ISEL on these
targets.

[hfinkel]

This patch is missing the parts that add the target feature and corresponding function.

lib/Target/PowerPC/PPCInstrInfo.cpp

bool PPCInstrInfo::canInsertSelect(const MachineBasicBlock &MBB,
   if (Cond.size() != 2)
     return false;
 
   // If this is really a bdnz-like condition, then it cannot be turned into a
   // select.
   if (Cond[1].getReg() == PPC::CTR || Cond[1].getReg() == PPC::CTR8)
     return false;
 
+  const MachineFunction *MF = MBB.getParent();
+  auto SelectPred = static_cast<PPC::Predicate>(Cond[0].getImm());
+  if (isHintedPredicate(SelectPred) &&
+      MF->getSubtarget<PPCSubtarget>().hasSlowISEL())
+    return false;
+
   // Check register classes.
   const MachineRegisterInfo &MRI = MBB.getParent()->getRegInfo();
   const TargetRegisterClass *RC =
     RI.getCommonSubClass(MRI.getRegClass(TrueReg), MRI.getRegClass(FalseReg));
   if (!RC)
     return false;
 
   // isel is for regular integer GPRs only.