RFR(S): 8237086: assert(is_MachReturn()) running CTW with fix for JDK-8231291

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8237086/webrev.00/

As discussed in:

https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2020-January/036689.html

GraphKit::must_be_not_null() is called with a known to be non null
input. This causes the null branch of the test to be optimized to top
and a top be added to the RootNode. RootNode::Ideal() is never called to
clean the top inputs and the assert is hit during matching.

Roland.