Do not set STMT_VINFO_VECTYPE for non-dataref stmts

Do not set STMT_VINFO_VECTYPE for non-dataref stmts

Now that all STMT_VINFO_VECTYPE uses from vectorizable_* have been
pruged there's no longer a need to have STMT_VINFO_VECTYPE set.
We still rely on it being present on data-ref stmts and there it
can differ between different SLP instances when doing BB vectorization.
The following removes the setting from vect_analyze_stmt and
vect_transform_stmt.

Note the following clears STMT_VINFO_VECTYPE from pattern stmts (the
vector type should have moved to the SLP tree by this time).

	* tree-vect-stmts.cc (vect_analyze_stmt): Only set
	STMT_VINFO_VECTYPE for dataref SLP representatives.
	Clear it for others and do not restore the original value.
	(vect_transform_stmt): Likewise.