[InstCombine] Remove an unnecessary dyn_cast to Instruction and a switch over two opcodes. Just dyn_cast to the specific instruction classes individually. NFC

Change the helper methods to take the more specific class as well.

llvm-svn: 311527
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index 65a08ce..6dcfade 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -687,9 +687,9 @@
   Instruction *foldICmpBinOp(ICmpInst &Cmp);
   Instruction *foldICmpEquality(ICmpInst &Cmp);
 
-  Instruction *foldICmpSelectConstant(ICmpInst &Cmp, Instruction *Select,
+  Instruction *foldICmpSelectConstant(ICmpInst &Cmp, SelectInst *Select,
                                       ConstantInt *C);
-  Instruction *foldICmpTruncConstant(ICmpInst &Cmp, Instruction *Trunc,
+  Instruction *foldICmpTruncConstant(ICmpInst &Cmp, TruncInst *Trunc,
                                      const APInt *C);
   Instruction *foldICmpAndConstant(ICmpInst &Cmp, BinaryOperator *And,
                                    const APInt *C);