cmd/compile: use keyed struct for asm tests

Just to make it clearer which regexps are positive and which
regexps are negative.

Change-Id: Ia190e89be28048fcae2491506f552afad90a5f85
Reviewed-on: https://go-review.googlesource.com/59490
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>