Simplify address arithmetic in vp9_optimize_b

Simplify address arithmetic in vp9_optimize_b

Simplify address arithmetic on token_costs to reduce the number of generated
instructions that are used for address arithmetic inside routine
vp9_optimize_b. It also helps improve instruction scheduling depending on
compiler and optimization level.

Measured a 9.3% reduction in retired instructions and 5.3% reduction in
execution time for this routine with GCC v4.8.4 and optimization flags -O3,
and a reduction of up to 11.6% in execution time with other compilers.

No change in behavior.

TEST=Verified that encoded files match bit for bit, with and without this
change.
BUG=b/33678225

Change-Id: I6098650fb5cd2aa04e014fe6e68ca20761f3a21f