RFR(S): 8203425: assert(is_Loop()) failed: invalid node class

[Nils Eliasson]

Hi,

This is a follow up to JDK-8203215 that caused failures in testing.

JDK-8203215 added a call to igvn.optimize after beautify_loops to handle 
phi-nodes that have lost their loop edge and now produce loop invariant 
data. In testing a failure came up - the igvn.optimize call can 
sometimes eliminate a loop node when the phis are all invariant.

When fixing this I observed that a loop might be removed, and that 
another region that is a subset of the first is selected as new loop, 
and then there might be new loop invariant phis (not observed).

This change introduces a fix point iteration for beautify_loops, until 
no more loop structure changes has been made. This might sound expensive 
but In practice beautify-loops is called one additional time at first 
insertion of loop-nodes. I have seen one case of cascading loop 
reductions when beautify_loops is called three times. The upside is that 
we reach fix point for PhaseIdealLoop faster since these changes would 
guarantee to trigger a major change in next iteration instead.

https://bugs.openjdk.java.net/browse/JDK-8203425

http://cr.openjdk.java.net/~neliasso/8203425/webrev.01/

Regards,

Nils Eliasson