Use higher byte of x86 16bits registers
Currently the register allocator only uses lower bytes of the aliased registers, AL/BL/CL/DL. The high byte (AH/BH/CH/DH) is wasted.
Currently the register allocator only uses lower bytes of the aliased registers, AL/BL/CL/DL. The high byte (AH/BH/CH/DH) is wasted.