Enable MachineOutliner by default under -Oz for AArch64

This patch enables the MachineOutliner by default in AArch64 under -Oz.

The MachineOutliner offers around a 4.5% improvement on the current -Oz code
size improvements.

We have done work into improving the debuggability of outlined code, so that
users of -Oz won't be surprised by the optimization. We have also been executing
the LLVM test suite and common external tests such as the SPEC suites
continuously with no issue. The outliner has a low compile-time overhead of
roughly 1%. At this point, the outliner would be a really good addition to the
-Oz pass pipeline!

llvm-svn: 338133