[BasicAA] Remove special casing of memset_pattern16 in favor of generic attribute inference

Most of the properties of memset_pattern16 can be now covered by the generic attributes and inferred by InferFunctionAttrs.  The only exceptions are:
- We don't yet have a writeonly attribute for the first argument.
- We don't have an attribute for modeling the access size facts encoded in MemoryLocation.cpp.  

Differential Revision: http://reviews.llvm.org/D15879

llvm-svn: 256911
diff --git a/llvm/test/Analysis/BasicAA/memset_pattern.ll b/llvm/test/Analysis/BasicAA/memset_pattern.ll
index 25bdb2e..821cbdf 100644
--- a/llvm/test/Analysis/BasicAA/memset_pattern.ll
+++ b/llvm/test/Analysis/BasicAA/memset_pattern.ll
@@ -18,4 +18,4 @@
   ret i32 %l
 }
 
-declare void @memset_pattern16(i8*, i8*, i64)
+declare void @memset_pattern16(i8*, i8* readonly, i64) argmemonly