V4 JIT: tune generated instructions for inplace binops

V4 JIT: tune generated instructions for inplace binops

Generate better code for in-place binary operations where the right-hand
side is either a constant or a memory address. Now that the JIT can do
this, also tell the register allocator not to un-spill that right-hand
side.

Change-Id: I0ab852f6b92f90dfed99c05fbaf91aad2549ecf4
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>