cmd/compile: fix linux-amd64-noopt builder

CL 312214 added unsafe.{Add,Slice}, which may appears in expression that
evaluating requires function calls. "mayCall" does not handle that
case, causing linux-amd64-noopt builder failed.

Fixes #45917

Change-Id: I900aefb702122480be621355def3d2c20eadbe70
Reviewed-on: https://go-review.googlesource.com/c/go/+/316170


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>