omp: Replace simd_clone_subparts with TYPE_VECTOR_SUBPARTS

Refactor simd clone handling code ahead of support for poly simdlen.

gcc/ChangeLog:

	* omp-simd-clone.cc (simd_clone_subparts): Remove.
	(simd_clone_init_simd_arrays): Replace simd_clone_supbarts with
	TYPE_VECTOR_SUBPARTS.
	(ipa_simd_modify_function_body): Likewise.
	* tree-vect-stmts.cc (vectorizable_simd_clone_call): Likewise.
	(simd_clone_subparts): Remove.