feat(amd64): assemble the double-shift and static-SB operand shapes

Assisted-by: GLM 5.3 Flash

asm/operand.go

@@ -48,3 +48,16 @@ type sbMem struct {
 }
 
 func (sbMem) isOperand() {}
+
+// isX86Mem reports whether the operand is an amd64 memory reference: a base
+// or indexed Mem, or an SB-relative sbMem.  Encoders that gate on "memory in
+// this position" must accept both; the r/m emitters distinguish the two
+// themselves.
+func isX86Mem(o Operand) bool {
+	switch o.(type) {
+	case Mem, sbMem:
+		return true
+	default:
+		return false
+	}
+}