RFR(XS): 8245714: "Bad graph detected in build_loop_late" when loads are pinned on loop limit check uncommon branch

[Roland Westrelin]

> https://bugs.openjdk.java.net/browse/JDK-8245714
> http://cr.openjdk.java.net/~roland/8245714/webrev.00/
>
> This triggers when data nodes are pinned on the uncommon trap path of a
> predicate. When a new predicate is added, a region is created to merge
> the paths comming from the place holder and the new predicate. Data
> nodes pinned on the uncommon path for the place holder are then updated
> to be pinned on the new region. That logic updates the control edge but
> not the control that loop opts keep track of. This causes a crash with
> the test case of the webrev where the predicate is a loop limit check.

That fix is incomplete. If the Load that's pinned on the uncommon trap
path is a LoadN then there's a DecodeN between the uncommon trap and the
Load. The control of the DecodeN also needs to be updated. Here is an
updated fix:

http://cr.openjdk.java.net/~roland/8245714/webrev.01/

This one uses lazy_replace. I'm concerned that other nodes (maybe an
AddP) would be assigned the projection as control and need to be
moved. With lazy_replace, all nodes are guaranteed to be properly
updated.

Roland.