tree-optimization/107672 - avoid vector mode type_for_mode call

The following avoids using type_for_mode on vector modes which might
not work for all frontends.  Instead we look for the inner mode
type and use build_vector_type_for_mode instead.

	PR tree-optimization/107672
	* tree-vect-stmts.cc (supportable_widening_operation): Avoid
	type_for_mode on vector modes.