cmd/compile: start using reflect.Value.IsZero

cmd/compile: start using reflect.Value.IsZero

We now require Go 1.17.13 to bootstrap via make.bash,
and since reflect.Value.IsZero was added in Go 1.13,
we can now use it directly to save a bit of copy pasting.

Change-Id: I77eef782cbbf86c72a4505c8b4866c9658914a24
Reviewed-on: https://go-review.googlesource.com/c/go/+/479395
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>