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

[Vladimir Ivanov]

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

Looks good.

PS: sorry for the late response, somehow I missed the initial request.

Best regards,
Vladimir Ivanov

> 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.
>