reg: support for register casting

Adds methods for referencing sub- or super-registers. For example, for
general purpose registers you can now reference As8(), As16(), ... and
for vector AsX(), AsY(), AsZ().

Closes #1

reg/x86_test.go

@@ -0,0 +1,28 @@
+package reg
+
+import "testing"
+
+func TestAsMethods(t *testing.T) {
+	cases := [][2]Register{
+		{RAX.As8(), AL},
+		{ECX.As8L(), CL},
+		{EBX.As8H(), BH},
+		{R9B.As16(), R9W},
+		{DH.As32(), EDX},
+		{R14L.As64(), R14},
+		{X2.AsX(), X2},
+		{X4.AsY(), Y4},
+		{X9.AsZ(), Z9},
+		{Y2.AsX(), X2},
+		{Y4.AsY(), Y4},
+		{Y9.AsZ(), Z9},
+		{Z2.AsX(), X2},
+		{Z4.AsY(), Y4},
+		{Z9.AsZ(), Z9},
+	}
+	for _, c := range cases {
+		if c[0] != c[1] {
+			t.FailNow()
+		}
+	}
+}