RFR(S): 8240335: C2: assert(found_sfpt) failed: no node in loop that's not input to safepoint

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8240335/webrev.00/

The node that is found in the outer strip mined loop but not referenced
from the safepoint is a CastII. The CastII node is there because on
unrolling of the inner loop, a region is created that merges the 2
copies of the body. The region is later optimized out by the IGVN but if
a Phi for that region is processed before, a cast node that cannot be
removed later on can be created. The fix I propose, is to not create the
cast if the phi's region or some of its inputs are still to be processed
by the IGVN.

Roland.