Completely rewrite support for the Value::use_* list.  Now, all operations on
this list (except use_size()) are constant time.  Before the killUse method
(used whenever something stopped using a value) was linear time, and thus
very very slow for large programs.

This speeds GCCAS up _substantially_ on large programs: almost 2x for 176.gcc:

176.gcc:     77.07s -> 37.38s
177.mesa:     7.59s ->  5.57s
252.eon:     21.02s -> 19.52s (*)
253.perlbmk: 11.40s -> 13.05s
254.gap:      7.25s -> 7.42s

252.eon would speed up a whole lot more, but optimization time is being
dominated by the inlining pass, which needs to be fixed.

llvm-svn: 9160