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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132695 91177308-0d34-0410-b5e6-96231b3b80d8