[CodeGen] Emit parallel_loop_access for each loop in the loop stack.

Summary:
Emit !llvm.mem.parallel_loop_access metadata for memory accesses even if the parallel loop is not the top on the loop stack.

Fixes llvm.org/PR37558.

Reviewers: ABataev, hfinkel, amusman, tyler.nowicki

Reviewed By: hfinkel

Subscribers: Meinersbur, hfinkel, cfe-commits

Differential Revision: https://reviews.llvm.org/D48808

llvm-svn: 338810