math/bits: use common mask constants

[bobinmad]

To simplify the code & increase performance up to 1%

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Keith Randall:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR is being closed because golang.org/cl/666335 has been abandoned.

Constants are executed at compile-time, if you are able to measure a 1% difference please open a bug with https://pkg.go.dev/golang.org/x/perf/cmd/benchstat results and a repro. This would be a bug in the compiler.