VAX: Fix predicates and constraints for bit-field comparison insns

VAX: Fix predicates and constraints for bit-field comparison insns

It makes no sense for insn operand predicates, as long as they accept a
register operand, to be more restrictive than the set of the associated
constraints, because expand will choose the insn based on the relevant
operand being a pseudo register then and reload keep it happily as a
memory reference if a constraint permits it.  So the restriction posed
by such a predicate will be happily ignored, and moreover if a splitter
is added, such as required for MODE_CC support, the new instructions
will reject the original operands supplied, causing an ICE.  An actual
example will be given with a subsequent change.

Therefore, similarly to EXTV/EXTZV/INSV insns, remove inconsistencies
with predicates and constraints of bit-field comparison insns, observing
that a bit-field located in memory is byte-addressed by the respective
machine instructions and therefore SImode may only be used with a
register or an offsettable memory operand (i.e. not an indexed,
pre-decremented, or post-incremented one).

Also give the insns names, for easier reference here and elsewhere.

	gcc/
	* config/vax/vax.md (*cmpv_2): Name insn.
	(*cmpv, *cmpzv, *cmpzv_2): Likewise.  Fix location predicate and
	constraint.