Fix incorrect bitshifting and masking in ffi bitfield (#10403)

When a uint8_t is bitshifted to the left, it is actually promoted to an
int. For the current code this has the effect of a

Fix incorrect bitshifting and masking in ffi bitfield (#10403)

When a uint8_t is bitshifted to the left, it is actually promoted to an
int. For the current code this has the effect of a wrong sign-extension,
and the result will also wrongly become zero when insert_pos >= 32.
Fix this by adding an explicit cast.
Furthermore, the partial prefix byte mask was computed incorrectly: the
byte is already shifted so the mask should not account for the shift.

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