[X86] Attempt to fix ubsan failure.

operator~ promote the single bit input to int. The ~ will cause the upper
31 bits to become 1s making it a negative value. This is undefined for
shift.

Mask it back down to a single bit.

The extra 1s were being shifted to bit 8 and above and the they aren't
used by the emitByte call so this shouldn't be a functional change.