Overhaul Lib/compiler block ordering. The previous code was filled with

hacks.  The new code is based on issue #2472 posted by Antoine Pitrou.  I
did some further cleanups of the pyassem code and optimized the block
ordering pass.

Lib/compiler/pycodegen.py

@@ -671,7 +671,7 @@ class CodeGenerator:
             self.startBlock(anchor)
             self.emit('POP_BLOCK')
             self.setups.pop()
-            self.startBlock(end)
+            self.nextBlock(end)
 
         self.emit('LOAD_CONST', None)