using frame pointer + constant in reg_equiv_constant

[Jeffrey A Law]

  In message < 199812010152.RAA03398@rtl.cygnus.com >you write:
  > With this patch, you should be able to revert your x86 LEGITIMATE_CONSTANT_P
  > change.  It might be useful to check which choice gives better code though,
  > and then document it in a comment.
Certainly worth trying.  I think the new LEGITIMATE_CONSTANT_P should generally
give better code, but I have no hard evidence to back up that assertion.

jeff