cmd/compile: remove manually implemented popcount in _gen/main.go
cmd/compile: remove manually implemented popcount in _gen/main.go

Change-Id: I7b4da26bace15c2b632c2dbfe56cc2799bbfdb02
Reviewed-on: https://go-review.googlesource.com/c/go/+/450058
Reviewed-by: Martin Möhrmann <9c82b83f348009b39856043bb9ed3ef41e171aeb@google.com>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@google.com>
Run-TryBot: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go