new coalescer causes too large stack frames

[Stephen Weeks]

I added some stuff to backend.fun so that it prints out the sizes of
stack frames.  I then ran a few tests, compiling hello-world.sml and
lexgen.sml using different coalesce thresholds.  The stack frame sizes
did not vary at all (as I would expect).  I think this makes it more
likely that the bug is not in the register allocator.  I am now more
interested than ever to play around with Henry's coalescer to see if
his coalescer causes the stack frame sizes to change.