pass: remove redundant jumps and dangling labels (#81)

In jump-table-like constructs, the natural way of writing the code can sometimes produce redundant jumps or labels. Therefore some basic cleanup steps have been proposed. This diff adds two transforms:

1. Remove unconditional jumps to a label immediately following.
2. Remove labels with no references at all.

Fixes #75

tests/labels/labels_test.go

@@ -0,0 +1,15 @@
+package labels
+
+import (
+	"testing"
+	"testing/quick"
+)
+
+//go:generate go run asm.go -out labels.s -stubs stub.go
+
+func TestLabels(t *testing.T) {
+	expect := func() uint64 { return 7 }
+	if err := quick.CheckEqual(Labels, expect, nil); err != nil {
+		t.Fatal(err)
+	}
+}