Teach basicaa that memset's modref set is at worst "mod" and never

contains "ref".

Enhance DSE to use a modref query instead of a store-specific hack
to generalize the "ignore may-alias stores" optimization to handle
memset and memcpy.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120368 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/BasicAliasAnalysis.cpp

@@ -703,6 +703,8 @@ BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
         if (isNoAlias(Location(Dest, Len), Loc))
           return NoModRef;
       }
+      // We know that memset doesn't load anything.
+      Min = Mod;
       break;
     case Intrinsic::atomic_cmp_swap:
     case Intrinsic::atomic_swap: