Introduce kHighRefcountMask which masks off flags and the LSb of the refcount value. In the cases where this mask is used, we don't need to check the LSb because we can assume the refcount is 1 when the rest of the masked RefcountAndFlags is empty, and the LSb doesn't matter if the masked value is not empty (either it's immortal or refcount > 1). This saves an instruction and a cycle (and + cmp -> tst) https://godbolt.org/z/Kz69eqfhq
PiperOrigin-RevId: 539151659
Change-Id: I2ec7d72918f052c4b0938edd746af9d5b3052c7e
