Improve jump threading 4 of N

[Jeff Law <law at redhat dot com>]

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This is merely an infrastructure patch in preparation for another
improvement in the jump threading code.  It shouldn't affect the code we
generate at all.

Let's consider this CFG (from a routine in cfgexpand.c)


           A
          / \
         B   C
         |  / \
         | D   E
         | |  / \
         | | F   G
          \| |
            \|
             H
            / \
           I   J
          / \
         L   M
         |  / \
         | N   O
         | |  / \
         | | P   Q
          \| |
            \|
             R


As it turns out some blocks have the same condition (A,I), (C,M), (E,O).
But because of the merge block H, no threading is possible.  What we
want to do is make 3 copies of H, each reachable from one predecessor of
the original H.

To do this we need to know the edge into the joiner block  so that we
can wire up the copy to the right predecessor.  We need the outgoing
edge from the joiner block that is threadable and finally we need the
destination edge.

This is a fairly simple extension to the existing code -- but we're
going to need the ability to store more information in the E->aux field.
 Previously E->aux stored the destination edge for a jump thread.   This
change allows E->aux to store additional data by changing it from a
single edge pointer to instead point to an array of edges.

FWIW, the patch to make copies of H to enable threading is probably the
limit of what can be done with the current threader.   I'm hoping to
start prototyping more generic path isolation code shortly.

Bootstrapped and regression tested on x86_64-unknown-linux-gnu.

OK for trunk?


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