Rework inReadableOrder() as a recursive descent.

Add a test for unreachable merge block.

Update test results with the new order: mainly delaying merge blocks and
removing unreachable ones.

Test/spv.merge-unreachable.frag

@@ -0,0 +1,7 @@
+#version 450
+layout(location=1) in highp vec4 v;
+void main (void)
+{
+  if (v == vec4(0.1,0.2,0.3,0.4)) discard;
+  else return;
+}