Add assume_safety option for pragma loop vectorize and interleave.

Add assume_safety option for pragma loop vectorize and interleave.

Specifying #pragma clang loop vectorize(assume_safety) on a loop adds the
mem.parallel_loop_access metadata to each load/store operation in the loop. This
metadata tells loop access analysis (LAA) to skip memory dependency checking.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239572 91177308-0d34-0410-b5e6-96231b3b80d8