[CodeGenPrep] Change ValueToSExts from DeseMap to MapVector

[HaohaiWen]

Summary

mergeSExts iterates throught ValueToSExts. Using DenseMap result in
unstable optimization path so that output IR may vary even if the input
IR is same.

[wxiao3]

LGTM.

llvm/lib/CodeGen/CodeGenPrepare.cpp

ModifyInstDT // Modify the Instruction Dominator in a Basic Block,
                // in a Basic Block. So we should re-iterate instructions
                // in such Basic Block.
 };
 
 using SetOfInstrs = SmallPtrSet<Instruction *, 16>;
 using TypeIsSExt = PointerIntPair<Type *, 2, ExtType>;
 using InstrToOrigTy = DenseMap<Instruction *, TypeIsSExt>;
 using SExts = SmallVector<Instruction *, 16>;
-using ValueToSExts = DenseMap<Value *, SExts>;
+using ValueToSExts = MapVector<Value *, SExts>;
 
 class TypePromotionTransaction;
 
 class CodeGenPrepare : public FunctionPass {
   const TargetMachine *TM = nullptr;
   const TargetSubtargetInfo *SubtargetInfo;
   const TargetLowering *TLI = nullptr;
   const TargetRegisterInfo *TRI;