[OPENMP 4.1] Add codegen for 'simdlen' clause.

Add emission of metadata for simd loops in presence of 'simdlen' clause.
If 'simdlen' clause is provided without 'safelen' clause, the vectorizer width for the loop is set to value of 'simdlen' clause + all read/write ops in loop are marked with '!llvm.mem.parallel_loop_access' metadata.
If 'simdlen' clause is provided along with 'safelen' clause, the vectorizer width for the loop is set to value of 'simdlen' clause + all read/write ops in loop are not marked with '!llvm.mem.parallel_loop_access' metadata.
If 'safelen' clause is provided without 'simdlen' clause, the vectorizer width for the loop is set to value of 'safelen' clause + all read/write ops in loop are not marked with '!llvm.mem.parallel_loop_access' metadata.

llvm-svn: 245697