When using GOEXPERIMENT=fieldtrack, we can see AUSEFIELD instructions.
We generally want to ignore them.

No tests because as far as I can tell there are no tests for
GOEXPERIMENT=fieldtrack.

Change-Id: Iee26f25592158e5db691a36cf8d77fc54d051314
Reviewed-on: https://go-review.googlesource.com/17610Reviewed-by: David Symonds <dsymonds@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>