middle-end/116083 - vectorizer slowness

middle-end/116083 - vectorizer slowness

Turns out SLP discovery can end up doing a lot of vector type
builds from scalar types.  Those are all ultimatively cached but
end up built and layouted first.  The latter is particularly
expensive because it does tree node arithmetic to compute TYPE_SIZE
and TYPE_SIZE_UNIT.  The following replaces this with the appropriate
poly-int arithmetic which speeds up the testcase by 50%.

	PR middle-end/116083
	* stor-layout.cc (layout_type): Compute TYPE_SIZE and
	TYPE_SIZE_UNIT for vector types from the component mode
	sizes.