There's nothing enforcing ordering between redundant nil checks when
they may occur during the same memory state. Commit to using the
earliest one in source order.

Otherwise the choice of which to remove depends on the ordering of
values in a block (before scheduling). That's unfortunate when trying
to ensure that the compiler doesn't depend on that ordering for
anything.

Update #20178

Change-Id: I2cdd5be10618accd9d91fa07406c90cbd023ffba
Reviewed-on: https://go-review.googlesource.com/c/151517
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>