Use a combination of table lookups and pshufb to convert coefficients
to zero run/level format. Two 16-entry lookup tables are used for a
total of 192 bytes worth of tables. (The existing SSE2 version uses a
table of size 2048 bytes.)
Speedup is ~1.5x-3x as compared with the SSE2 version on Haswell (the
speedup is greater for input with many trailing zeros).
The use of popcnt makes it require SSE4.2. This can be replaced with
a small LUT and accumulation which would reduce the requirement to
SSSE3.
