Refactor HGraphBuilder and SsaBuilder to remove HLocals

This patch merges the instruction-building phases from HGraphBuilder
and SsaBuilder into a single HInstructionBuilder class. As a result,
it is not necessary to generate HLocal, HLoadLocal and HStoreLocal
instructions any more, as the builder produces SSA form directly.

Saves 5-15% of arena-allocated memory (see bug for more data):
  GMS      20.46MB  =>  19.26MB  (-5.86%)
  Maps     24.12MB  =>  21.47MB  (-10.98%)
  YouTube  28.60MB  =>  26.01MB  (-9.05%)

Bug: 27894376
Change-Id: Iefe28d40600c169c5d306fd2c77034ae19476d90