[tree-ssa] Add description for dominator walker

[law@redhat.com]

In message <1081375964.3172.566.camel@localhost.localdomain>, Diego Novillo wri
tes:
 >
 >A brief overview of the dominator walker.
 >
 >
 >Diego.
 >
 >
 >	* doc/tree-ssa.texi: Add documentation for the dominator
 >	walker.
Note that I've also got an overview here of how to use the dominator walker
to build an optimizer -- which also happens to give a simplistic overview
of the dominator optimizer itself.


jeff