RFR(M): 8212610: Fix handling of memory in PhaseIdealLoop::clone_loop_predicates()

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8212610/webrev.00/

I had previously sent this as part of 8212243.

We have:

Predicate 1
..
Predicate n

profile predicate 1
..
profile predicate n

loop head

Shenandoah write barriers produce memory, can be hoisted out of loop but
can also depend on some predicate so may end up with:

Predicate 1
..
Predicate n

profile predicate 1
..
write barrier
...
profile predicate n

loop head

In that case, all predicates branch to a region that dominates the
uncommon trap call. Same is true for profile predicates. Because of the
write barrier, the region for profile predicates has a memory phi to
merge memory state before the barrier and after the barrier. Now when
clone_loop_predicates() is called:

Predicate 1
..
Predicate n

profile predicate 1
..
write barrier
...
profile predicate n

cloned predicate

cloned profile predicate

loop head

the cloned predicate branches to the region shared by all
predicates. Same goes for the cloned profile predicate and the profile
predicates. The memory state for the cloned predicate should be the one
following the barrier but after cloning it's the one from the predicate
n, before the barrier. That memory state needs to be adjusted.

Roland.