RFR(XS) 8068864: C2 failed: modified node is not on IGVN._worklist

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/8068864/webrev/

Use igvn.replace_input_of() instead of set_req(). Same problem and the 
same place as in 8053915.

Very complex code during javac compilation which cause a particular 
order of igvn._worklist processing. I can't write a test.

Tested with JPRT.

Thanks,
Vladimir