pass: allow consecutive labels (#123)

Fixes #122

tests/fixedbugs/issue122/issue122.s

@@ -0,0 +1,32 @@
+// Code generated by command: go run asm.go -out issue122.s -stubs stub.go. DO NOT EDIT.
+
+#include "textflag.h"
+
+// func Triangle(n uint64) uint64
+TEXT ·Triangle(SB), NOSPLIT, $0-16
+	MOVQ n+0(FP), AX
+
+	// Initialize sum register to zero.
+	XORQ CX, CX
+
+loop_even:
+loop_odd:
+	// Loop until n is zero.
+	CMPQ AX, $0x00
+	JE   done
+
+	// Add n to sum.
+	ADDQ AX, CX
+
+	// Decrement n.
+	DECQ AX
+
+	// Jump to one of the loop labels depending on parity.
+	TESTQ $0x00000001, AX
+	JZ    loop_even
+	JMP   loop_odd
+
+done:
+	// Store sum to return value.
+	MOVQ CX, ret+8(FP)
+	RET