[NFC][PowerPC] Simplify the logic in lower select_cc

[steven.zhang]

Summary

The logic in select_cc is messy and hard to follow. This is a NFC patch to simplify the logic.

[ZhangKang]

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

7543–7547

Above comments should be splitted into 2 parts
One is below, it should before the line 7569.

// We might be able to do better than this under some circumstances, but in
  // general, fsel-based lowering of select is a finite-math-only optimization.
  // For more information, see section F.3 of the 2.06 ISA specification.

The other is below, it should before the line 7556.

// With ISA 3.0, we have xsmaxcdp/xsmincdp which are OK to emit even in the
  // presence of infinities.

You patch is similar without the patch https://reviews.llvm.org/D62993 .

[steven.zhang]

Update the comments.

[ZhangKang]

LGTM.

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

 /// LowerSELECT_CC - Lower floating point select_cc's into fsel instruction when
 /// possible.
 SDValue PPCTargetLowering::LowerSELECT_CC(SDValue Op, SelectionDAG &DAG) const {
   // Not FP? Not a fsel.
   if (!Op.getOperand(0).getValueType().isFloatingPoint() ||
       !Op.getOperand(2).getValueType().isFloatingPoint())
     return Op;
 
-  bool HasNoInfs = DAG.getTarget().Options.NoInfsFPMath;
-  bool HasNoNaNs = DAG.getTarget().Options.NoNaNsFPMath;
   // We might be able to do better than this under some circumstances, but in
   // general, fsel-based lowering of select is a finite-math-only optimization.
   // For more information, see section F.3 of the 2.06 ISA specification.
   // With ISA 3.0, we have xsmaxcdp/xsmincdp which are OK to emit even in the
   // presence of infinities.
-  if (!Subtarget.hasP9Vector() && (!HasNoInfs || !HasNoNaNs))
-    return Op;
   ISD::CondCode CC = cast<CondCodeSDNode>(Op.getOperand(4))->get();
 
   EVT ResVT = Op.getValueType();
   EVT CmpVT = Op.getOperand(0).getValueType();
   SDValue LHS = Op.getOperand(0), RHS = Op.getOperand(1);
   SDValue TV  = Op.getOperand(2), FV  = Op.getOperand(3);
   SDLoc dl(Op);
 
   if (Subtarget.hasP9Vector() && LHS == TV && RHS == FV) {
     switch (CC) {
     default:
-      // Not a min/max but with finite math, we may still be able to use fsel.
-      if (HasNoInfs && HasNoNaNs)
-        break;
-      return Op;
     case ISD::SETOGT:
     case ISD::SETGT:
       return DAG.getNode(PPCISD::XSMAXCDP, dl, Op.getValueType(), LHS, RHS);
     case ISD::SETOLT:
     case ISD::SETLT:
       return DAG.getNode(PPCISD::XSMINCDP, dl, Op.getValueType(), LHS, RHS);
     }
-  } else if (!HasNoInfs || !HasNoNaNs)
+  }
+
+  if (!DAG.getTarget().Options.NoInfsFPMath ||
+      !DAG.getTarget().Options.NoNaNsFPMath)
     return Op;
 
   // TODO: Propagate flags from the select rather than global settings.
   SDNodeFlags Flags;
   Flags.setNoInfs(true);
   Flags.setNoNaNs(true);
 
   // If the RHS of the comparison is a 0.0, we don't need to do the