[IR] SelectInst: add swapValues() utility

[lebedev.ri]

Summary

Sometimes we need to swap true-val and false-val of a SelectInst.
Having a function for that is nicer than hand-writing it each time.

[jdoerfert]

LGTM

llvm/include/llvm/IR/Instructions.h

public:
   Value *getCondition() { return Op<0>(); }
   Value *getTrueValue() { return Op<1>(); }
   Value *getFalseValue() { return Op<2>(); }
 
   void setCondition(Value *V) { Op<0>() = V; }
   void setTrueValue(Value *V) { Op<1>() = V; }
   void setFalseValue(Value *V) { Op<2>() = V; }
 
+  /// Swap the true and false values of the select instruction.
+  /// This doesn't swap prof metadata.
+  void swapValues() { Op<1>().swap(Op<2>()); }
+
   /// Return a string if the specified operands are invalid
   /// for a select operation, otherwise return null.
   static const char *areInvalidOperands(Value *Cond, Value *True, Value *False);
 
   /// Transparently provide more efficient getOperand methods.
   DECLARE_TRANSPARENT_OPERAND_ACCESSORS(Value);
 
   OtherOps getOpcode() const {

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

if (Op0C->isNullValue()) {
       }
 
       if (Op1->hasOneUse()) {
         Value *LHS, *RHS;
         SelectPatternFlavor SPF = matchSelectPattern(Op1, LHS, RHS).Flavor;
         if (SPF == SPF_ABS || SPF == SPF_NABS) {
           // This is a negate of an ABS/NABS pattern. Just swap the operands
           // of the select.
-          SelectInst *SI = cast<SelectInst>(Op1);
-          Value *TrueVal = SI->getTrueValue();
-          Value *FalseVal = SI->getFalseValue();
-          SI->setTrueValue(FalseVal);
-          SI->setFalseValue(TrueVal);
+          cast<SelectInst>(Op1)->swapValues();
           // Don't swap prof metadata, we didn't change the branch behavior.
-          return replaceInstUsesWith(I, SI);
+          return replaceInstUsesWith(I, Op1);
         }
       }
     }
 
     // Turn this into a xor if LHS is 2^n-1 and the remaining bits are known
     // zero.
     if (Op0C->isMask()) {
       KnownBits RHSKnown = computeKnownBits(Op1, 0, &I);

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

canonicalizeMinMaxWithConstant(SelectInst &Sel, ICmpInst &Cmp,
 
   // If the select operands did not change, we're done.
   if (Sel.getTrueValue() == LHS && Sel.getFalseValue() == RHS)
     return &Sel;
 
   // If we are swapping the select operands, swap the metadata too.
   assert(Sel.getTrueValue() == RHS && Sel.getFalseValue() == LHS &&
          "Unexpected results from matchSelectPattern");
-  Sel.setTrueValue(LHS);
-  Sel.setFalseValue(RHS);
+  Sel.swapValues();
   Sel.swapProfMetadata();
   return &Sel;
 }
 
 /// There are many select variants for each of ABS/NABS.
 /// In matchSelectPattern(), there are different compare constants, compare
 /// predicates/operands and select operands.
 /// In isKnownNegation(), there are different formats of negated operands.
if (SPF == SelectPatternFlavor::SPF_NABS) {
     assert(FVal == LHS && "Unexpected results from matchSelectPattern");
   } else {
     if (FVal == LHS)
       return &Sel;
     assert(TVal == LHS && "Unexpected results from matchSelectPattern");
   }
 
   // We are swapping the select operands, so swap the metadata too.
-  Sel.setTrueValue(FVal);
-  Sel.setFalseValue(TVal);
+  Sel.swapValues();
   Sel.swapProfMetadata();
   return &Sel;
 }
 
 /// Visit a SelectInst that has an ICmpInst as its first operand.
 Instruction *InstCombiner::foldSelectInstWithICmp(SelectInst &SI,
                                                   ICmpInst *ICI) {
   Value *TrueVal = SI.getTrueValue();

llvm/lib/Transforms/Instrumentation/ControlHeightReduction.cpp

if (auto *BI = dyn_cast<BranchInst>(U)) {
       // TrueBiasedRegions/FalseBiasedRegions because true-based/false-based
       // mean whehter the branch is likely go into the if-then rather than
       // successor0/successor1 and because we can tell which edge is the then or
       // the else one by comparing the destination to the region exit block.
       continue;
     }
     if (auto *SI = dyn_cast<SelectInst>(U)) {
       // Swap operands
-      Value *TrueValue = SI->getTrueValue();
-      Value *FalseValue = SI->getFalseValue();
-      SI->setTrueValue(FalseValue);
-      SI->setFalseValue(TrueValue);
+      SI->swapValues();
       SI->swapProfMetadata();
       if (Scope->TrueBiasedSelects.count(SI)) {
         assert(Scope->FalseBiasedSelects.count(SI) == 0 &&
                "Must not be already in");
         Scope->FalseBiasedSelects.insert(SI);
       } else if (Scope->FalseBiasedSelects.count(SI)) {
         assert(Scope->TrueBiasedSelects.count(SI) == 0 &&
                "Must not be already in");