[InstCombine] use loop instead of recursion to peek through FPExt; NFCI

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295992 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -1223,17 +1223,15 @@ static Constant *fitsInFPType(ConstantFP *CFP, const fltSemantics &Sem) {
   return nullptr;
 }
 
-/// If this is a floating-point extension instruction, look
-/// through it until we get the source value.
+/// Look through floating-point extensions until we get the source value.
 static Value *lookThroughFPExtensions(Value *V) {
-  if (Instruction *I = dyn_cast<Instruction>(V))
-    if (I->getOpcode() == Instruction::FPExt)
-      return lookThroughFPExtensions(I->getOperand(0));
+  while (auto *FPExt = dyn_cast<FPExtInst>(V))
+    V = FPExt->getOperand(0);
 
   // If this value is a constant, return the constant in the smallest FP type
   // that can accurately represent it.  This allows us to turn
   // (float)((double)X+2.0) into x+2.0f.
-  if (ConstantFP *CFP = dyn_cast<ConstantFP>(V)) {
+  if (auto *CFP = dyn_cast<ConstantFP>(V)) {
     if (CFP->getType() == Type::getPPC_FP128Ty(V->getContext()))
       return V;  // No constant folding of this.
     // See if the value can be truncated to half and then reextended.