[IR] Mark memset.* intrinsics as IntrWriteMem.

[IR] Mark memset.* intrinsics as IntrWriteMem.

llvm.memset intrinsics do only write memory, but are missing
IntrWriteMem, so they doesNotReadMemory() returns false for them.

The test change is due to the test checking the fn attribute ids at the
call sites, which got bumped up due to a new combination with writeonly
appearing in the test file.

Reviewers: jdoerfert, reames, efriedma, nlopes, lebedev.ri

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D72789