[bugpoint] Add support for -Oz and properly enable -Os.
This patch adds -Oz as option and also properly enables support for -Os.
Currently, the existing check for -Os is dead, because the enclosing if
only checks of O1, O2 and O3.
There is still a difference between the -Oz pipeline compared to opt,
but I have not been able to track that down yet.
Reviewers: bogner, sebpop, efriedma
Reviewed By: efriedma
Differential Revision: https://reviews.llvm.org/D67593