use asm_offsets with vp8_regular_quantize_b_sse2

remove helper function and avoid shadowing all the arguments to the
stack on 64bit systems

when running with --good --cpu-used=0:
~2% on linux x86 and x86_64
~2% on win32 x86 msys and visual studio
more on darwin10 x86_64
significantly more on
x86_64-win64-vs9

Change-Id: Ib7be12edf511fbf2922f191afd5b33b19a0c4ae6