Fix vectorizable_conversion costs

Fix vectorizable_conversion costs

This patch makes two tweaks to vectorizable_conversion.  The first
is to use "modifier" to distinguish between promotion, demotion,
and neither promotion nor demotion, rather than using a code for
some cases and "modifier" for others.  The second is to take ncopies
into account for the promotion and demotion costs; previously we gave
multiple copies the same cost as a single copy.

Later patches test this, but it seemed worth splitting out.

2019-11-13  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (vect_model_promotion_demotion_cost): Take the
	number of ncopies as an additional argument.
	(vectorizable_conversion): Update call accordingly.  Use "modifier"
	to check whether a conversion is between vectors with the same
	numbers of units.

From-SVN: r278121