Ilya added POPCNT in a CL earlier this month but it's really only POPCNTQ.
The other forms still need to be added.

For #4816.

Change-Id: I1186850d32ad6d5777475c7808e6fc9d9133e118
Reviewed-on: https://go-review.googlesource.com/18848Reviewed-by: Rob Pike <r@golang.org>