patch for update_flow_info REG_WAS_0

[Joern Rennecke]

> One might argue that the note should have been removed when the
> insn was deleted.  Can you look into that?

I first looked at the REG_WAS_0 description in rtl.texi.  It appears that
is a known fact that the insn might be deleted.  So I presume that that's
OK, and we have to deal with it.

@findex REG_WAS_0
@item REG_WAS_0
The single output of this insn contained zero before this insn.
@var{op} is the insn that set it to zero.  You can rely on this note if
it is present and @var{op} has not been deleted or turned into a @code{note};
its absence implies nothing.
@end table

These notes describe linkages between insns.  They occur in pairs: one
insn has one of a pair of notes that points to a second insn, which has
the inverse note pointing back to the first insn.