CS_MergeLabels: Keep labels referenced by data

Partial fix for ICE in #1211.  This may fix enough to allow #1049 to be
fixed.

When merging labels, keep the first label with a ref that has no JumpTo;
this is a data segment label, used by computed gotos.

The real fix is to track and rewrite labels in data, but this is more
involved.

test/val/bug1211-ice-move-refs-1.c

@@ -0,0 +1,53 @@
+/*
+  Copyright 2020 The cc65 Authors
+
+  This software is provided 'as-is', without any express or implied
+  warranty. In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+/*
+  Test of indirect goto with label merge ICE.
+  https://github.com/cc65/cc65/issues/1211
+  This test case works because CS_MergeLabels has a hack to keep the "unreferenced" label
+  (i.e. the one referenced in a data segment).
+*/
+
+#include <stdio.h>
+
+/* When operating correctly, f(0) = 31 and f(1) = 41. */
+int f (int x)
+{
+    static const void *const labels[] = {&&L0, &&L1};
+    goto *labels[x];
+L0: if (labels[0] != labels[1]) return 31;
+    else return 13;
+L1: return 41;
+}
+
+static unsigned char failures = 0;
+
+int main (void)
+{
+    if (f (0) != 31) failures++;
+
+    if (failures == 0) {
+        printf ("PASS\n");
+    } else {
+        printf ("FAIL\n");
+    }
+
+    return failures;
+}