autovectorizer: Add basic support for convert optabs

Given the shift from modeling dot products as direct optabs to
treating them as conversion optabs, we make necessary changes to the
autovectorizer code to ensure that given the relevant tree code,
together with the input and output data modes, we can retrieve the
relevant optab and subsequently the insn_code for it.

gcc/ChangeLog:

	* gimple-match-exports.cc (directly_supported_p): Add overload
	for conversion-type optabs.
	* gimple-match.h (directly_supported_p): Add new function
	prototype.
	* optabs.cc (expand_widen_pattern_expr): Make the
	DOT_PROD_EXPR tree code use `find_widening_optab_handler' to
	retrieve icode.
	* tree-vect-loop.cc (vect_is_emulated_mixed_dot_prod): make it
	call conversion-type overloaded `directly_supported_p'.
	* tree-vect-patterns.cc (vect_supportable_conv_optab_p): New.
	(vect_recog_dot_prod_pattern): s/direct/conv/ in call to
	`vect_supportable_direct_optab_p'.