Updated the llvm.mem.parallel_loop_access semantics to include the possibility

Updated the llvm.mem.parallel_loop_access semantics to include the possibility
to have only some of the loop's memory instructions be annotated and still _help_
the loop carried dependence analysis.

This was discussed in the llvmdev ML (topic: "parallel loop metadata question").

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209507 91177308-0d34-0410-b5e6-96231b3b80d8