[InstCombine] refactor trunc(binop) transforms; NFCI

In addition to moving the shift transforms over, we may want to
detect too-wide rotate patterns here (PR34046). 

llvm-svn: 310181
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index e3dfc66..f550dd5 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -439,7 +439,7 @@
   Instruction *scalarizePHI(ExtractElementInst &EI, PHINode *PN);
   Value *EvaluateInDifferentElementOrder(Value *V, ArrayRef<int> Mask);
   Instruction *foldCastedBitwiseLogic(BinaryOperator &I);
-  Instruction *shrinkBitwiseLogic(TruncInst &Trunc);
+  Instruction *narrowBinOp(TruncInst &Trunc);
   Instruction *optimizeBitCastFromPhi(CastInst &CI, PHINode *PN);
 
   /// Determine if a pair of casts can be replaced by a single cast.