VAX: Also provide QImode and HImode `ctz' and `ffs' operations

VAX: Also provide QImode and HImode `ctz' and `ffs' operations

The FFS machine instruction provides for arbitrary input bit-field widths
so take advantage of this and convert `ffssi2' and `ctzsi2' to templates
for all the three of QI, HI, SI machine modes.

Test cases will be added separately.

	gcc/
	* config/vax/builtins.md (width): New mode attribute.
	(ffssi2): Rework expander into...
	(ffs<mode>2): ... this.
	(ctzsi2): Rework insn into...
	(ctz<mode>2): ... this.