[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

llvm-svn: 372079