Teach the 'opt' tool about '-Os' and '-Oz', corresponding to the Clang

options, to enable easier testing of the innards of LLVM that are
enabled by such optimization strategies.

Note that this doesn't provide the (much needed) function attribute
support for -Oz (as opposed to -Os), but still seems like a positive
step to better test the logic that Clang currently relies on.

Patch by Patrik Hägglund.

llvm-svn: 156913