Fix compiler warning

Fix compiler warning

Introduced in 0aa8f7640.

MSVC warned about performing 32-bit bit shifting when it appeared like we
might like a 64-bit result.  We did, but it just so happened that none of
the calls to this function could have caused the 32-bit shift to overflow.

Here we just cast the constant to int64 to make the compiler happy.

Discussion: https://postgr.es/m/CAApHDvofA_vsrpC13mq_hZyuye5B-ssKEaer04OouXYCO5-uXQ@mail.gmail.com