Fix memcmp code-gen to honor -fno-builtin.

I noticed that SelectionDAGBuilder::visitCall was missing a check for memcmp
in TargetLibraryInfo, so that it would use custom code for memcmp calls even
with -fno-builtin.  I also had to add a new -disable-simplify-libcalls option
to llc so that I could write a test for this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161262 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -5682,7 +5682,7 @@ void SelectionDAGBuilder::visitCall(const CallInst &I) {
                                    Tmp.getValueType(), Tmp));
           return;
         }
-      } else if (Name == "memcmp") {
+      } else if ((LibInfo->has(LibFunc::memcmp) && Name == "memcmp")) {
         if (visitMemCmpCall(I))
           return;
       }