tests/fixedbugs: regression test for issue 100 (#129)

Adds a regression test based on klauspost/compress#186. This necessitated some related changes:

* Mark "RET" as a terminal instruction
* printer refactor to maintain compatibility with asmfmt
* Tweaks to other regression tests to ensure they are run correctly in CI

Updates #100 #65 #8

pass/cfg.go

@@ -62,7 +62,7 @@ func CFG(fn *ir.Function) error {
 		// Otherwise, could continue to the following instruction.
 		switch {
 		case cur.IsTerminal:
-		case cur.IsBranch && !cur.IsConditional:
+		case cur.IsUnconditionalBranch():
 		default:
 			cur.Succ = append(cur.Succ, nxt)
 		}