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

ir/ir.go

@@ -61,6 +61,11 @@ type Instruction struct {
 
 func (i *Instruction) node() {}
 
+// IsUnconditionalBranch reports whether i is an unconditional branch.
+func (i Instruction) IsUnconditionalBranch() bool {
+	return i.IsBranch && !i.IsConditional
+}
+
 // TargetLabel returns the label referenced by this instruction. Returns nil if
 // no label is referenced.
 func (i Instruction) TargetLabel() *Label {