[BasicAA] Delete dead code related to memset/memcpy/memmove intrinsics [NFCI]

[reames]

Summary

We only need to describe the writeonly property of one of the arguments. All of the rest of the semantics are nicely described by existing attributes in Intrinsics.td.

lib/Analysis/BasicAliasAnalysis.cpp

ModRefInfo BasicAAResult::getArgModRefInfo(ImmutableCallSite CS,
                                            unsigned ArgIdx) {
   if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction()))
     switch (II->getIntrinsicID()) {
     default:
       break;
     case Intrinsic::memset:
     case Intrinsic::memcpy:
     case Intrinsic::memmove:
-      assert((ArgIdx == 0 || ArgIdx == 1) &&
-             "Invalid argument index for memory intrinsic");
-      return ArgIdx ? MRI_Ref : MRI_Mod;
+      // We don't currently have a writeonly attribute.  All other properties
+      // of these intrinsics are nicely described via attributes in
+      // Intrinsics.td and handled generically below.
+      if (ArgIdx == 0)
+        return MRI_Mod;
     }
 
   // We can bound the aliasing properties of memset_pattern16 just as we can
   // for memcpy/memset.  This is particularly important because the
   // LoopIdiomRecognizer likes to turn loops into calls to memset_pattern16
   // whenever possible.
   if (CS.getCalledFunction() &&
       isMemsetPattern16(CS.getCalledFunction(), TLI)) {