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/stub.go

@@ -0,0 +1,5 @@
+// Code generated by command: go run asm.go -out labels.s -stubs stub.go. DO NOT EDIT.
+
+package labels
+
+func Labels() uint64