for review (M): 6812831 factor megamorphic invokeinterface (for 6655638)

[John Rose]

In response to a review request (thanks, Vladimir!) I've split the  
refactoring of the itable lookup logic into a separate change request.

Here it is; please (re-)review:
  http://cr.openjdk.java.net/~jrose/6812831/webrev

It is a little different from the corresponding changes in the mega- 
review.
I cleaned it up slightly, so that the interface search loop is  
tighter.  (Should fit on an I$ line now.)
I also make it work on x86 (32 and 64) and SPARC.

-- John