Backport r69961 to trunk, replacing JUMP_IF_{TRUE,FALSE} with

POP_JUMP_IF_{TRUE,FALSE} and JUMP_IF_{TRUE,FALSE}_OR_POP. This avoids executing
a POP_TOP on each conditional and sometimes allows the peephole optimizer to
skip a JUMP_ABSOLUTE entirely. It speeds up list comprehensions significantly.

Lib/compiler/pyassem.py

@@ -122,6 +122,9 @@ def order_blocks(start_block, exit_block):
     for b in remaining:
         if __debug__ and b.next:
             assert b is b.next[0].prev[0], (b, b.next)
+        # Make sure every block appears in dominators, even if no
+        # other block must precede it.
+        dominators.setdefault(b, set())
         # preceeding blocks dominate following blocks
         for c in b.get_followers():
             while 1: