Found looking at mapaccess1_faststr.

runtime.throw never returns.
Do x+y+c with an LEA.

Change-Id: I27ea6669324242a6302397cbdc73230891d97591
Reviewed-on: https://go-review.googlesource.com/19911
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>