[FunctionAttrs] Remove readonly and writeonly assertion

There are scenarios where mutually recursive functions may cause the SCC
to contain both read only and write only functions. This removes an
assertion when adding read attributes which caused a crash with a the
provided test case, and instead just doesn't add the attributes.

Patch by Luke Lau <luke.lau@intel.com>

Differential Revision: https://reviews.llvm.org/D60761

llvm-svn: 366090