cmd/compile: remove implicit deref from len(p) where p is ptr-to-array

cmd/compile: remove implicit deref from len(p) where p is ptr-to-array

func f() *[4]int { return nil }
_ = len(f())

should not panic. We evaluate f, but there isn't a dereference
according to the spec (just "arg is evaluated").

Update #72844

Change-Id: Ia32cefc1b7aa091cd1c13016e015842b4d12d5b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/658096
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Keith Randall <khr@google.com>