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_test.go

@@ -0,0 +1,58 @@
+package pass
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/mmcloughlin/avo/ir"
+	"github.com/mmcloughlin/avo/operand"
+	"github.com/mmcloughlin/avo/reg"
+)
+
+func TestVerifyMemOperands(t *testing.T) {
+	i := &ir.Instruction{
+		Operands: []operand.Op{
+			reg.RAX,
+			operand.Mem{
+				Base: reg.R10,
+				Disp: 42,
+			},
+		},
+	}
+	if err := VerifyMemOperands(i); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestVerifyMemOperandsErrors(t *testing.T) {
+	cases := []struct {
+		Operands       []operand.Op
+		ErrorSubstring string
+	}{
+		{
+			Operands: []operand.Op{
+				reg.RAX,
+				operand.Mem{
+					Disp: 42,
+				},
+			},
+			ErrorSubstring: "missing base",
+		},
+		{
+			Operands: []operand.Op{
+				operand.Mem{
+					Base:  reg.EBX,
+					Index: reg.R9L,
+				},
+				reg.ECX,
+			},
+			ErrorSubstring: "index register with scale 0",
+		},
+	}
+	for _, c := range cases {
+		i := &ir.Instruction{Operands: c.Operands}
+		if err := VerifyMemOperands(i); err == nil || !strings.Contains(err.Error(), c.ErrorSubstring) {
+			t.Errorf("got error %v; expected error to contain %q", err, c.ErrorSubstring)
+		}
+	}
+}