pass: VerifyMemOperands (#127)

Introduces a pass to verify memory operands. This is added to a new
Verify pass, that in future could do much more.

Updates #125

pass/verify.go

@@ -0,0 +1,32 @@
+package pass
+
+import (
+	"errors"
+
+	"github.com/mmcloughlin/avo/ir"
+	"github.com/mmcloughlin/avo/operand"
+)
+
+// Verify pass validates an avo file.
+var Verify = Concat(
+	InstructionPass(VerifyMemOperands),
+)
+
+// 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 {
+			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
+}