[InstCombine] reduce duplicate code; NFC

An unused variable problem was introduced with rL352870 
and stubbed out with rL352871, but we can make a better
fix by actually using the local variable in code rather 
than just the assert.  

llvm-svn: 352873
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index caed651..811b0fd 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -761,7 +761,6 @@
   assert(RetTy->getStructElementType(0)->isIntegerTy(8) &&
          RetTy->getStructElementType(1) == OpTy && OpTy == Op2->getType() &&
          "Unexpected types for x86 addcarry");
-  (void)RetTy;
 
   // If carry-in is zero, this is just an unsigned add with overflow.
   if (match(CarryIn, m_ZeroInt())) {
@@ -771,7 +770,7 @@
     Value *UAddResult = Builder.CreateExtractValue(UAdd, 0);
     Value *UAddOV = Builder.CreateZExt(Builder.CreateExtractValue(UAdd, 1),
                                        Builder.getInt8Ty());
-    Value *Res = UndefValue::get(II.getType());
+    Value *Res = UndefValue::get(RetTy);
     Res = Builder.CreateInsertValue(Res, UAddOV, 0);
     return Builder.CreateInsertValue(Res, UAddResult, 1);
   }