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

tests/fixedbugs/issue100/minrepro/asm.go

@@ -0,0 +1,22 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+	. "github.com/mmcloughlin/avo/operand"
+)
+
+func main() {
+	TEXT("Issue100", NOSPLIT, "func() uint64")
+	x := GP64()
+	XORQ(x, x)
+	for i := 1; i <= 100; i++ {
+		t := GP64()
+		MOVQ(U32(i), t)
+		ADDQ(t.As64(), x)
+	}
+	Store(x, ReturnIndex(0))
+	RET()
+	Generate()
+}