[LoopDeletion] Move ICmpInst handling to getValueOnFirstIteration()

As noticed in https://reviews.llvm.org/D105688, it would be great to move
handling of ICmpInst which was in canProveExitOnFirstIteration() to
getValueOnFirstIteration().

Patch by Dmitry Makogon!

Differential Revision: https://reviews.llvm.org/D108978
Reviewed By: reames

llvm/lib/Transforms/Scalar/LoopDeletion.cpp

@@ -191,6 +191,12 @@ getValueOnFirstIteration(Value *V, DenseMap<Value *, Value *> &FirstIterValue,
     Value *RHS =
         getValueOnFirstIteration(BO->getOperand(1), FirstIterValue, SQ);
     FirstIterV = SimplifyBinOp(BO->getOpcode(), LHS, RHS, SQ);
+  } else if (auto *Cmp = dyn_cast<ICmpInst>(V)) {
+    Value *LHS =
+        getValueOnFirstIteration(Cmp->getOperand(0), FirstIterValue, SQ);
+    Value *RHS =
+        getValueOnFirstIteration(Cmp->getOperand(1), FirstIterValue, SQ);
+    FirstIterV = SimplifyICmpInst(Cmp->getPredicate(), LHS, RHS, SQ);
   }
   if (!FirstIterV)
     FirstIterV = V;
@@ -314,22 +320,20 @@ static bool canProveExitOnFirstIteration(Loop *L, DominatorTree &DT,
     }
 
     using namespace PatternMatch;
-    ICmpInst::Predicate Pred;
-    Value *LHS, *RHS;
+    Value *Cond;
     BasicBlock *IfTrue, *IfFalse;
     auto *Term = BB->getTerminator();
-    if (match(Term, m_Br(m_ICmp(Pred, m_Value(LHS), m_Value(RHS)),
+    if (match(Term, m_Br(m_Value(Cond),
                          m_BasicBlock(IfTrue), m_BasicBlock(IfFalse)))) {
-      if (!LHS->getType()->isIntegerTy()) {
+      auto *ICmp = dyn_cast<ICmpInst>(Cond);
+      if (!ICmp || !ICmp->getType()->isIntegerTy()) {
         MarkAllSuccessorsLive(BB);
         continue;
       }
 
       // Can we prove constant true or false for this condition?
-      LHS = getValueOnFirstIteration(LHS, FirstIterValue, SQ);
-      RHS = getValueOnFirstIteration(RHS, FirstIterValue, SQ);
-      auto *KnownCondition = SimplifyICmpInst(Pred, LHS, RHS, SQ);
-      if (!KnownCondition) {
+      auto *KnownCondition = getValueOnFirstIteration(ICmp, FirstIterValue, SQ);
+      if (KnownCondition == ICmp) {
         // Failed to simplify.
         MarkAllSuccessorsLive(BB);
         continue;