RFR(S): 8238384: CTW: C2 compilation fails with "assert(store != load->find_exact_control(load->in(0))) failed: dependence cycle found"

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8238384/webrev.00/

In the test case: the newly allocated array of the first copyOf doesn't
escape and is assigned an instance id. The memory edge of the element
load for that array is modified to skip over the copyOf. C2 can't
eliminate the non escaping allocations and the element load. When
anti-dependencies are added, the load is found to be both control
dependent on the membar that was added when the second copyOf is
expanded and anti dependent on it: the assert triggers.

The bug is similar to 8228772 that Tobias fixed last summer but I
propose a different fix. I think the root cause is that known instances
don't alias with bottom memory. That's what allows memory edges for
known instances to be rewired. So I propose Compile::can_alias() be
changed to reflect that. AFAICT, ArrayCopy is an exception (it can copy
to a known instance but has adr_type, the bottom type) for which I've
made some changes.

Roland.