Minor SSA cleanups

[law@redhat.com]

  In message < 871yof95z9.fsf@cgsoftware.com >you write:
  > Jeffrey A Law <law@localhost.localdomain> writes:
  > 
  > > We want remove_phi_alternative to have external scope so that SSA optimiz
  > ers
  > > which remove unexecutable edges can also remove phi alternatives.  
  > Weird.
  > You seem to be doing exactly the same patches I did.
  > 
  > I also global scoped phi_alternative.
  > 
  > The other change to ssa.c I have is:
  > 
  > *************** mark_phi_and_copy_regs (phi_set)
  > *** 1959,1964 ****
  > --- 1995,2002 ----
  >         if (insn == NULL)
  >           continue;
  >         pattern = PATTERN (insn);
  > +       if (!pattern)
  > +         continue;
  >         /* Sometimes we get PARALLEL insns.  These aren't phi nodes or
  >            copies.  */
  >         if (GET_CODE (pattern) != SET)
I don't think you want/need this.  This should be caught further upstream.

  > Since the SSA definition array isn't kept in sync after each pass, if
  > you've removed a register, the insn in the ssa_defs isn't necessarily
  > an INSN anymore.  But it won't be NULL. It could be a deleted NOTE or
  > something weird.
It should be a NOTE_INSN_DELETED.  Anything else indicates a bug elsewhere.


  > > We're
  > > also going to be changing the PHI nodes to record block pointers rather
  > > than block indices (but that'll be a separate patch).
  > 
  > Why *do* we have block indices or pointers at all there?
  > They seem unecessary, especially given that a register can only have
  > one def. So to figure out what block it came from, ....
It's significantly simpler to just record where it came from when you
generate the PHI node.

jeff