test: remove amd64 build tag from test

It was supposed to be testing SSA, not amd64.

For #18024

Change-Id: Ibe65d7eb6bed9bc4b3eda68e1eaec5fa39fe8f76
Reviewed-on: https://go-review.googlesource.com/33491Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue8613.go

-// +build amd64
 // run
 
 // Copyright 2016 The Go Authors. All rights reserved.