Almost never happens in practice.
The compiler will generate reasonable code anyway,
since assignments involving [0]T never do any work.

Fixes #19696
Fixes #19671

Change-Id: I350d2e0c5bb326c4789c74a046ab0486b2cee49c
Reviewed-on: https://go-review.googlesource.com/38599
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>