This reverts commit 2dc83299.

The commit made jumpthreading "more conservative", which fixed the
problem. However, it is not a good solution in general.

We need a bigger rewrite of opt/jumpthreading, so it analyzes then
modifies the graph instead of on-the-fly changes. This prevents the
infinite loop, which would e.g. happen with such a cf graph:

    \     A         \     A
   __\    |        __\    |
   |  B   |   =>   |  B   |  => ...
   | / \  |        | / \  |
   |    D |        |    E |
   |     \|        |     \|
    \     C         \     D
     \___/           \   /
                      \_C

Breaks opt/jumpthreading5 and opt/jumpthreading5b