Improve vector_vector_composition_type

Improve vector_vector_composition_type

We sometimes get to ask to decompose, say V2DFmode into two halves.
Currently this results in composing it from two DImode pieces
instead of the obvious two DFmode pieces.  The following adjusts
vector_vector_composition_type for this trivial case and avoids
a VIEW_CONVERT_EXPR in the initial code generation.

	* tree-vect-stmts.cc (vector_vector_composition_type):
	Handle composition of a vector from a number of elements that
	happens to match its number of lanes.