arm: add smax/smin expanders for v*hf

This patch adds the missing expanders for smax/smin for v*hf modes,
by using the VDQWH iterator instead of VALLW.

2022-09-08  Christophe Lyon  <christophe.lyon@arm.com>

	gcc/
	* config/arm/vec-common.md (smin<mode>3): Use VDQWH iterator.
	(smax<mode>3): Likewise.