This is a patch for PRs 68471 and 68472, which show problems with the 
ROP mitigation:
  * reg-stack doesn't call df_insn_update when it makes changes, and
    if df checking is enabled, any subsequent df_analyze call will
    abort
  * Using -mcmodel=medium fails because of a pattern that has lea type
    and needs its modrm_class overridden.

Both of these are fixed in the i386 backend. As a further safety 
measure, I've added some extra code to regrename to ignore stack regs 
after regstack_complete - they can't be dealt with anymore.

Bootstrapped and tested on x86_64-linux, with -mmitigate-rop forced on. Ok?


Bernd