SSE2, msac: Use bsr shortcut for 50% bool decoding
SSE2, msac: Use bsr shortcut for 50% bool decoding

bsr has 3 cycles of latency for modern x86 processors. For this
function, it's possible to obtain the number of bits to shift by
alternative means.

I'd estimate about approx -0.2% decrease in cpu usage based on
percentages associated with function symbols in perf report.
Benchmarks were run on a Ryzen 5 3600 (Zen 2). The used clip was the
original 1080p chimera.

xiph/rav1e,xiph/rav1e