Allow user to use a Mem with a nil Base

[cocotyty]

It's very useful for users who want to leverage LEAQ to speed up some integer computations.

For example:

LEAQ -8(AX * 8), BX

is much efficiency than:

MOVQ $+8,BX
MULQ BX
SUBQ 8, AX
MOVQ AX,BX

[cocotyty]

@mmcloughlin

avo/operand/checks.go

Line 189 in 3bfcefe

func IsMSize(op Op, n uint) bool {

// IsMSize returns true if op is a memory operand using general-purpose address
// registers of the given size in bytes.
func IsMSize(op Op, n uint) bool {
	// TODO(mbm): should memory operands have a size attribute as well?
	// TODO(mbm): m8,m16,m32,m64 checks do not actually check size
	m, ok := op.(Mem)
	return ok && IsMReg(m.Base) && (m.Index == nil || IsMReg(m.Index))
}

I think it is not very hard to implement this feature.

[klauspost]

A PR would make the change easy to add.

Meanwhile I wouldn't expect XORQ(tmp, tmp) to be a huge overhead and use that as base. For most CPUs zeroing a register is basically ignored, since they just use a virtual zero register.