Replace mode_for_int_vector with related_int_vector_mode

Replace mode_for_int_vector with related_int_vector_mode

mode_for_int_vector, like mode_for_vector, can sometimes return
an integer mode or an unsupported vector mode.  But no callers
are interested in that case, and only want supported vector modes.
This patch therefore replaces mode_for_int_vector with
related_int_vector_mode, which gives the target a chance to pick
its preferred vector mode for the given element mode and size.

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

gcc/
	* machmode.h (mode_for_int_vector): Delete.
	(related_int_vector_mode): Declare.
	* stor-layout.c (mode_for_int_vector): Delete.
	(related_int_vector_mode): New function.
	* optabs.c (expand_vec_perm_1): Use related_int_vector_mode
	instead of mode_for_int_vector.
	(expand_vec_perm_const): Likewise.
	* config/aarch64/aarch64.c (aarch64_emit_approx_sqrt): Likewise.
	(aarch64_evpc_sve_tbl): Likewise.
	* config/s390/s390.c (s390_expand_vec_compare_cc): Likewise.
	(s390_expand_vcond): Likewise.

From-SVN: r278230