SCATTER/GATHER Vector memory operand validation is too strict

[vsivsi]

E.g. The following instruction is perfectly valid AVX-512: VPSCATTERQQ Z7, K1, (Z8*1)

Avo currently enforces that all memory operands have a non-nil GP64 base register.

However for the VM(64/32)(x/y/z) memory operands used by the scatter/gather instructions, these checks are too strict, because as the above instruction shows, the Base register is optional for VM* memory operands.

I've tentatively modified the functions isvm() and VerifyMemOperands(...) to:

func isvm(op Op, idx func(Op) bool) bool {
	m, ok := op.(Mem)
	// return ok && IsR64(m.Base) && idx(m.Index)
	return ok && (m.Base == nil || IsR64(m.Base)) && idx(m.Index)
}

And:

// VerifyMemOperands checks the instruction's memory operands.
func VerifyMemOperands(i *ir.Instruction) error {
	for _, op := range i.Operands {
		m, ok := op.(operand.Mem)
		if !ok {
			continue
		}

		// if m.Base == nil {
		if m.Base == nil && !(operand.IsVmx(m) || operand.IsVmy(m) || operand.IsVmz(m)) {
			return errors.New("bad memory operand: missing base register")
		}

		if m.Index != nil && m.Scale == 0 {
			return errors.New("bad memory operand: index register with scale 0")
		}
	}
	return nil
}

This seems to do the trick. I keep meaning to put together a PR with the AVX512_BITALG new instructions: VPOPCNTB/W and VPSHUFBITQMB. If the above fixes look okay, I'll fold them in as well.