tweak atomic-fetch-bit-op mask casts
tweak atomic-fetch-bit-op mask casts

Summary: Cast the result of shifts to the integer type parameter, rather than the shift width.

Reviewed By: aary, luciang

Differential Revision: D32160950

fbshipit-source-id: 0100a1a92c48bfa1cd957cf42ed84d01302ea4f7

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