[WIP][InstCombine] See if we can determine the value of a use based on a dominating condition

[mcrosier]

Summary

Test case:

int dataflow(int b) {
  int a;

  if (b==4)
    a = 3*b; // fully optimized when a is simplified to a constant;
  else 
    a = 5;

  if (a == 4)
    return 0;
  else
    return 1;
}

As an example, we can replace the use of 'b' in 'a = 3 * b' with '4', which then allows the function to simplify to just 'return 1'.

This is just something I quickly threw together. If anyone has interest in this I can add test cases and submit for formal review.

Chad

lib/Transforms/InstCombine/InstructionCombining.cpp

if (match(&BI, m_Br(m_OneUse(m_Cmp(Pred, m_Value(), m_Value())), TrueDest,
     // Swap destinations and condition.
     CmpInst *Cond = cast<CmpInst>(BI.getCondition());
     Cond->setPredicate(CmpInst::getInversePredicate(Pred));
     BI.swapSuccessors();
     Worklist.Add(Cond);
     return &BI;
   }
 
+  // See if we can determine the value of a use based on a dominating condition.
+  Value *Op0;
+  ConstantInt *C;
+  if (match(&BI, m_Br(m_ICmp(Pred, m_Value(Op0), m_ConstantInt(C)), TrueDest,
+                      FalseDest)) &&
+      (Pred == ICmpInst::ICMP_EQ || Pred == ICmpInst::ICMP_NE) &&
+      Op0->getNumUses() >= 2) {
+    BasicBlock *TargetBB = Pred == ICmpInst::ICMP_EQ ? TrueDest : FalseDest;
+    if (TargetBB->getSinglePredecessor()) {
+      // This branch dominates TargetBB and we known that Op0 == C in TargetBB.
+      for (User *U : Op0->users())
+        if (Instruction *UI = dyn_cast<Instruction>(U))
+          if (UI->getParent() == TargetBB)
+            for (unsigned OpNum = 0; OpNum < UI->getNumOperands(); ++OpNum)
+              if (UI->getOperand(OpNum) == Op0)
+                U->setOperand(OpNum, C);
+
+      // FIXME: We've changed things, but not the branch. IC need to know this.
+    }
+  }
+
   return nullptr;
 }
 
 Instruction *InstCombiner::visitSwitchInst(SwitchInst &SI) {
   Value *Cond = SI.getCondition();
   Value *Op0;
   ConstantInt *AddRHS;
   if (match(Cond, m_Add(m_Value(Op0), m_ConstantInt(AddRHS)))) {