New bugpoint feature

I've just commited a small patch that gives bugpoint the smarts to run
the optimizations through valgrind when searching for errors.

Pass "--enable-valgrind" to bugpoint and it will use Valgrind to test
for memory errors when minimizing a testcase. When using this, you might
also want to pass the -timeout=0 option, as Valgrind is about 25-50x
slower than native. (The default timeout is 5 minutes.)

Nick Lewycky