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/fmt/fmt.s

@@ -13,4 +13,5 @@ TEXT ·Formatting(SB), NOSPLIT, $0
 label:
 	// Comment after label.
 	ADDQ R8, R8
+	JMP  label
 	RET