Make less use of get_same_sized_vectype

Make less use of get_same_sized_vectype

Some callers of get_same_sized_vectype were dealing with operands that
are constant or defined externally, and so have no STMT_VINFO_VECTYPE
available.  Under the current model, using get_same_sized_vectype for
that case is equivalent to using get_vectype_for_scalar_type, since
get_vectype_for_scalar_type always returns vectors of the same size,
once a size is fixed.

Using get_vectype_for_scalar_type is arguably more obvious though:
if we're using the same scalar type as we would for internal
definitions, we should use the same vector type too.  (Constant and
external definitions sometimes let us change the original scalar type
to a "nicer" scalar type, but that isn't what's happening here.)

This is a prerequisite to supporting multiple vector sizes in the same
vec_info.

2019-11-14  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (vectorizable_call): If an operand is
	constant or external, use get_vectype_for_scalar_type
	rather than get_same_sized_vectype to get its vector type.
	(vectorizable_conversion, vectorizable_shift): Likewise.
	(vectorizable_operation): Likewise.

From-SVN: r278238