patch to warn_uninit

[Per Bothner]

Per Bothner wrote:
> An alternative is to pass the actual tree node in the calls to
> walk_node, rather than the locus - something like the attached
> tree-ssa.alt-patch.  I haven't verified that this alternative
> patch works (except that it compiles), but I can if that is
> deemed nicer.

I did a bootrap+check with the alternative patch, with
--enable-mapped-location, and it seems to work fine.
(I would of course alo check --disable-maped-location before
checking it in.)
-- 
	--Per Bothner
per@bothner.com   http://per.bothner.com/