omp-simd-clone: Allow fixed-lane vectors

omp-simd-clone: Allow fixed-lane vectors

The vecsize_int/vecsize_float has an assumption that all arguments will use
the same bitsize, and vary the number of lanes according to the element size,
but this is inappropriate on targets where the number of lanes is fixed and
the bitsize varies (i.e. amdgcn).

With this change the vecsize can be left zero and the vectorization factor will
be the same for all types.

gcc/ChangeLog:

	* doc/tm.texi: Regenerate.
	* omp-simd-clone.cc (simd_clone_adjust_return_type): Allow zero
	vecsize.
	(simd_clone_adjust_argument_types): Likewise.
	* target.def (compute_vecsize_and_simdlen): Document the new
	vecsize_int and vecsize_float semantics.