RFR(S): 8186716: [MVT] C2 crashes with "assert(!n->is_ValueType()) failed: value types should not be split through phis"

[Roland Westrelin]

> http://cr.openjdk.java.net/~thartmann/8186716/webrev.00/

That looks reasonable to me. Why isn't a GrowableArray good enough
anymore to keep track of value type nodes?

Roland.