[IR] Remove createReplacementInstr. NFC.

[foad]

Summary

It is unused since D112791.

[hsmhsm]

LGTM

llvm/include/llvm/IR/ReplaceConstant.h

 
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Instruction.h"
 #include <map>
 #include <vector>
 
 namespace llvm {
 
-/// Create a replacement instruction for constant expression \p CE and insert
-/// it before \p Instr.
-Instruction *createReplacementInstr(ConstantExpr *CE, Instruction *Instr);
-
 /// The given instruction \p I contains given constant expression \p CE as one
 /// of its operands, possibly nested within constant expression trees. Convert
 /// all reachable paths from contant expression operands of \p I to \p CE into
 /// corresponding instructions, insert them before \p I, update operands of \p I
 /// accordingly, and if required, return all such converted instructions at
 /// \p Insts.
 void convertConstantExprsToInstructions(
     Instruction *I, ConstantExpr *CE,

llvm/lib/IR/ReplaceConstant.cpp

 //===----------------------------------------------------------------------===//
 
 #include "llvm/IR/ReplaceConstant.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/NoFolder.h"
 
 namespace llvm {
-// Replace a constant expression by instructions with equivalent operations at
-// a specified location.
-Instruction *createReplacementInstr(ConstantExpr *CE, Instruction *Instr) {
-  return CE->getAsInstruction(Instr);
-}
 
 void convertConstantExprsToInstructions(Instruction *I, ConstantExpr *CE,
                                         SmallPtrSetImpl<Instruction *> *Insts) {
   // Collect all reachable paths to CE from constant exprssion operands of I.
   std::map<Use *, std::vector<std::vector<ConstantExpr *>>> CEPaths;
   collectConstantExprPaths(I, CE, CEPaths);
 
   // Convert all constant expressions to instructions which are collected at