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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256911 91177308-0d34-0410-b5e6-96231b3b80d8