GCC build failed with your patch on 2001-02-17T20:05:01Z.

[Richard Kenner]

    /sloth/delay/tbox/cvs-gcc/gcc/gcc/libgcc2.c: In function `__divdi3':
    /sloth/delay/tbox/cvs-gcc/gcc/gcc/libgcc2.c:747: Unrecognizable insn:
    (insn 725 722 195 (set (reg:SI 339)
            (and:SI (reg/v:SI 125)
                (const_int 65535 [0xffff]))) -1 (nil)
        (nil))

    Something's not checking that insns are valid after making
    substitutions.  This insn is invalid because it needs an extra
    CLOBBER.

I verified apply_change_grouup is called.  I guess I have to get unlazy
and make an RS6000 cross-compiler ...