Changed dx register allocator to place more phis and reuse locals.

Changed dx register allocator to place more phis and reuse locals.

This change makes the register allocator try to place all phis earlier,
by using whichever register is most common among its sources and result.

In addition, the code tries much harder to reuse registers originally
reserved for locals if they are no longer live. This leads to fewer
registers in many methods.

Change-Id: I5f69320686184f784384f5cf3a1d9c97e44ec19d