Handle variable-length vectors in compute_record_mode

This patch makes compute_record_mode handle SVE vectors in the
same way as it would handle fixed-length vectors.  There should
be no change in behaviour for other targets.

This is needed for the SVE equivalent of arm_neon.h types like
int8x8x2_t (i.e. a pair of int8x8_ts).

2019-09-18  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* stor-layout.c (compute_record_mode): Operate on poly_uint64
	sizes instead of uhwi sizes.

From-SVN: r275869