Add mode_for_int_vector helper functions

Add mode_for_int_vector helper functions

There are at least a few places that want to create an integer vector
with a specified element size and element count, or to create the
integer equivalent of an existing mode.  This patch adds helpers
for doing that.

The require ()s are all used in functions that go on to emit
instructions that use the result as a vector mode.

2017-09-05  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* machmode.h (mode_for_int_vector): New function.
	* stor-layout.c (mode_for_int_vector): Likewise.
	* config/aarch64/aarch64.c (aarch64_emit_approx_sqrt): Use it.
	* config/powerpcspe/powerpcspe.c (rs6000_do_expand_vec_perm): Likewise.
	* config/rs6000/rs6000.c (rs6000_do_expand_vec_perm): Likewise.
	* config/s390/s390.c (s390_expand_vec_compare_cc): Likewise.
	(s390_expand_vcond): Likewise.

From-SVN: r251729