Prevent combine_instructions() from following non-existant LOG_LINKS

[Jeffrey A Law]

  In message <200001261857.KAA27932@elmo.cygnus.com>you write:
  > Hi Guys,
  > 
  >   I ran into a problem with combine_instructions() when RTL checking
  >   is enabled.  Sometimes it can try to combine an insn which has been
  >   deleted and the RTL checking code compainins about an attempt to
  >   access the LOG_LINKS of a NOTE which is not supposed to have such a
  >   field.  (It does have one, because it once was an insn, but the RTL
  >   checking code does not know this).  The patch below fixes this small
  >   problem by tetsing to see if the insn is a NOTE before LOG_LINKS is
  >   called.
  > 
  >   OK to apply this patch ?
  > 
  > Cheers
  > 	Nick
  > 
  > 2000-01-25  Nick Clifton  <nickc@redhat.com>
  > 
  > 	* combine.c (combine_instructions): Do not try to combine a
  > 	sequence of insns when the second insn has been replaced by a
  > 	note.
This is fine.  Go ahead and install it.

Note that at some point we may be able to undo this change if we twiddle
the auto-inc code in the right manner.  But until then I don't see a reason
why we should continue triggering an enable-checking failure.

jeff