pass: allow consecutive labels (#123)

Fixes #122

tests/fixedbugs/issue122/asm.go

@@ -0,0 +1,42 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+	. "github.com/mmcloughlin/avo/operand"
+)
+
+func main() {
+	TEXT("Triangle", NOSPLIT, "func(n uint64) uint64")
+	Doc("Triangle computes the nth triangle number.")
+	n := Load(Param("n"), GP64())
+
+	Comment("Initialize sum register to zero.")
+	s := GP64()
+	XORQ(s, s)
+
+	// Use two labels for the top of the loop.
+	Label("loop_even")
+	Label("loop_odd")
+	Comment("Loop until n is zero.")
+	CMPQ(n, Imm(0))
+	JE(LabelRef("done"))
+
+	Comment("Add n to sum.")
+	ADDQ(n, s)
+
+	Comment("Decrement n.")
+	DECQ(n)
+
+	Comment("Jump to one of the loop labels depending on parity.")
+	TESTQ(U32(1), n)
+	JZ(LabelRef("loop_even"))
+	JMP(LabelRef("loop_odd"))
+
+	Label("done")
+	Comment("Store sum to return value.")
+	Store(s, ReturnIndex(0))
+	RET()
+	Generate()
+}