[AArch64][GlobalISel] Enable the localizer for optimized builds.
Despite the fact that the localizer's original motivation was to fix horrendous
constant spilling at -O0, shortening live ranges still has net benefits even
with optimizations enabled.
On an -Os build of CTMark, doing this improves code size by 0.5% geomean.
There are a few regressions, bullet increasing in size by 0.5%. One example from
bullet where code size increased slightly was due to GlobalISel actually now
generating the same code as SelectionDAG. So we actually have an opportunity
in future to implement better heuristics for localization and therefore be
*better* than SDAG in some cases. In relation to other optimizations though that
one is relatively minor.
Differential Revision: https://reviews.llvm.org/D67303