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

ir/ir_test.go

@@ -0,0 +1,22 @@
+package ir
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestFunctionLabels(t *testing.T) {
+	f := NewFunction("labels")
+	f.AddInstruction(&Instruction{})
+	f.AddLabel("a")
+	f.AddInstruction(&Instruction{})
+	f.AddLabel("b")
+	f.AddInstruction(&Instruction{})
+	f.AddLabel("c")
+	f.AddInstruction(&Instruction{})
+
+	expect := []Label{"a", "b", "c"}
+	if got := f.Labels(); !reflect.DeepEqual(expect, got) {
+		t.Fatalf("f.Labels() = %v; expect %v", got, expect)
+	}
+}