Remove the MarkModRef pass (use AddReadAttrs instead).

Unfortunately this means removing one regression test
of GlobalsModRef because I couldn't work out how to
perform it without MarkModRef.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56342 91177308-0d34-0410-b5e6-96231b3b80d8