poly_int: get_mask_mode

This patch makes TARGET_GET_MASK_MODE take polynomial nunits and
vector_size arguments.  The gcc_assert in default_get_mask_mode
is now handled by the exact_div call in vector_element_size.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* target.def (get_mask_mode): Take the number of units and length
	as poly_uint64s rather than unsigned ints.
	* targhooks.h (default_get_mask_mode): Update accordingly.
	* targhooks.c (default_get_mask_mode): Likewise.
	* config/i386/i386.c (ix86_get_mask_mode): Likewise.
	* doc/tm.texi: Regenerate.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r256130