SLSR: Use UnknownAddressSpace instead of default

[arsenm]

Summary

In the case where isLegalAddressingMode is used for cases
not related to addressing modes, such as pure adds and muls,
it should not be using address space 0. LSR already passes -1
as the address space in these cases.

[jingyue]

Maybe change the title to be "... instead of default for pure arithmetic instructions". Otherwise, LGTM!

[arsenm]

r267645

lib/Transforms/Scalar/StraightLineStrengthReduce.cpp

 #include "llvm/Transforms/Scalar.h"
 #include "llvm/Transforms/Utils/Local.h"
 
 using namespace llvm;
 using namespace PatternMatch;
 
 namespace {
 
+static const unsigned UnknownAddressSpace = ~0u;
+
 class StraightLineStrengthReduce : public FunctionPass {
 public:
   // SLSR candidate. Such a candidate must be in one of the forms described in
   // the header comments.
   struct Candidate : public ilist_node<Candidate> {
     enum Kind {
       Invalid, // reserved for the default constructor
       Add,     // B + i * S
static bool isGEPFoldable(GetElementPtrInst *GEP,
   return TTI->isLegalAddressingMode(GEP->getResultElementType(), BaseGV,
                                     BaseOffset, HasBaseReg, Scale, AddrSpace);
 }
 
 // Returns whether (Base + Index * Stride) can be folded to an addressing mode.
 static bool isAddFoldable(const SCEV *Base, ConstantInt *Index, Value *Stride,
                           TargetTransformInfo *TTI) {
   return TTI->isLegalAddressingMode(Base->getType(), nullptr, 0, true,
-                                    Index->getSExtValue());
+                                    Index->getSExtValue(), UnknownAddressSpace);
 }
 
 bool StraightLineStrengthReduce::isFoldable(const Candidate &C,
                                             TargetTransformInfo *TTI,
                                             const DataLayout *DL) {
   if (C.CandidateKind == Candidate::Add)
     return isAddFoldable(C.Base, C.Index, C.Stride, TTI);
   if (C.CandidateKind == Candidate::GEP)