Add -O1/2/3 to bugpoint, so when you conclude opt -O2 reproduces an issue, you...

Add -O1/2/3 to bugpoint, so when you conclude opt -O2 reproduces an issue, you can just run bugpoint -O2. :)  My implementation isn't precisely equivalent to what opt does, but as far as I can tell, it's close enough.

llvm-svn: 132695