[AST] Fix a bug in aliasesUnknownInst. Make sure we are comparing the unknown instructions in the alias set and the instruction interested in.

[trentxintong]

Summary

Make sure we are comparing the unknown instructions in the alias set and the instruction interested in.
I believe this is clearly a bug (missed opportunity). I can also add some test cases if desired.

[davide]

Yes, please add some tests.

[trentxintong]

Add test.

[trentxintong]

Add comments

[trentxintong]

Add 1 more assert in the test case.

[hfinkel]

LGTM

lib/Analysis/AliasSetTracker.cpp

bool AliasSet::aliasesUnknownInst(const Instruction *Inst,
 
   if (AliasAny)
     return true;
 
   if (!Inst->mayReadOrWriteMemory())
     return false;
 
   for (unsigned i = 0, e = UnknownInsts.size(); i != e; ++i) {
-    if (auto *Inst = getUnknownInst(i)) {
-      ImmutableCallSite C1(Inst), C2(Inst);
+    if (auto *UnknownInst = getUnknownInst(i)) {
+      ImmutableCallSite C1(UnknownInst), C2(Inst);
       if (!C1 || !C2 || AA.getModRefInfo(C1, C2) != MRI_NoModRef ||
           AA.getModRefInfo(C2, C1) != MRI_NoModRef)
         return true;
     }
   }
 
   for (iterator I = begin(), E = end(); I != E; ++I)
     if (AA.getModRefInfo(Inst, MemoryLocation(I.getPointer(), I.getSize(),