scc constraint removal can produce empty WL block!

Bugzilla Link	756
Created on	Oct 03, 2010 21:45
Version	svn
OS	Linux
Architecture	PC
Attachments	crud.sac

Extended Description

In the attached, compiling with:
 sac2c-d crud.sac -O3 -doawlf -nowlf -extrema -ecc  -v1
for build #developer rev 17069:MODIFIED 
causes a crash in EBT after SCC. It looks like the problem
is that SCC blindly eliminates code from a block, but
this can (and does, in this case) result it removing the
last piece of code from an N_with code block, which is
verboten.
I think the fix is to put some code in SCCblock to check
for this and insert the canonical EMPTY marker if the
block does become empty.