[RFA] three simple combine cleanups

[David Edelsohn]

	One of your recent combine.c patches is causing a miscompilation
of f951 on powerpc-linux and powerpc-aix.  See
http://gcc.gnu.org/ml/fortran/2005-12/msg00487.html for specific examples.
The symptom is an incorrect simplification performed in the gfortran
front-end which is visible in the Fortran parse tree dump output.
Reverting your patch and rebootstrapping produces correct Fortran parse
tree output.  Graham's patch does not fix the problem.

David