cmd/compile: move unsafe operation out of EvalConst

cmd/compile: move unsafe operation out of EvalConst

So future CLs can get rid of EvalConst entirely.

Change-Id: Ic8e147fd76e53c002a6ceda2fb3be979459bf865
Reviewed-on: https://go-review.googlesource.com/c/go/+/484315
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>