openmp: fix max_vf setting for amdgcn offloading

openmp: fix max_vf setting for amdgcn offloading

Ensure that the "max_vf" figure used for the "safelen" attribute is large
enough for the largest configured offload device.

This change gives ~10x speed improvement on the Bablestream "dot" benchmark for
AMD GCN.

gcc/ChangeLog:

	* gimple-loop-versioning.cc (loop_versioning::loop_versioning): Add
	comment.
	* omp-general.cc (omp_max_simd_vf): New function.
	* omp-general.h (omp_max_simd_vf): New prototype.
	* omp-low.cc (lower_rec_simd_input_clauses): Select largest from
	  omp_max_vf, omp_max_simt_vf, and omp_max_simd_vf.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp
	(check_effective_target_amdgcn_offloading_enabled): New.
	(check_effective_target_nvptx_offloading_enabled): New.
	* gcc.dg/gomp/target-vf.c: New test.