Use-and-clobber insns and some ancient combine.c code

[Richard Kenner]

    I guess I can see why we wouldn't want to substitute into the operand
    of a CLOBBER itself.  But why prevent substitution of an expression
    containing a register that will be clobbered?  This second clause was
    added under this ChangeLog entry:

I don't remember the second issue, but as to this one, I think the
point is that you can't selectively substitute into the insn, so you
would update the CLOBBER.  But I'm not sure.