This patch makes omp_max_vf return a polynomial vectorization factor.
We then need to be able to stash a polynomial value in
OMP_CLAUSE_SAFELEN_EXPR too:

   /* If max_vf is non-zero, then we can use only a vectorization factor
      up to the max_vf we chose.  So stick it into the safelen clause.  */

For now the cfgloop safelen is still constant though.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* omp-general.h (omp_max_vf): Return a poly_uint64 instead of an int.
	* omp-general.c (omp_max_vf): Likewise.
	* omp-expand.c (omp_adjust_chunk_size): Update call to omp_max_vf.
	(expand_omp_simd): Handle polynomial safelen.
	* omp-low.c (omplow_simd_context): Add a default constructor.
	(omplow_simd_context::max_vf): Change from int to poly_uint64.
	(lower_rec_simd_input_clauses): Update accordingly.
	(lower_rec_input_clauses): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r256129