[release-branch.go1.13] cmd/compile: remove check that Zero's arg has the correct base type

It doesn't have to. The type in the aux field is authoritative.
There are cases involving casting from interface{} where pointers
have a placeholder pointer type (because the type is not known when
the IData op is generated).

The check was introduced in CL 13447.

Fixes #39848

Change-Id: Id77a57577806a271aeebd20bea5d92d08ee7aa6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/239817


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
(cherry picked from commit 3b2f67a5)
Reviewed-on: https://go-review.googlesource.com/c/go/+/239998
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>