Negative-case conversion code was wrong for minimum int32,
used negate-then-widen instead of widen-then-negate.

Test already exists; this fixes the failure.

Fixes #15563.

Change-Id: I4b0b3ae8f2c9714bdcc405d4d0b1502ccfba2b40
Reviewed-on: https://go-review.googlesource.com/22830
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>