Run live vars test only on ssa builds.
We can't just drop KeepAlive ops during regalloc.  We need
to replace them with copies.

Change-Id: Ib4b3b1381415db88fdc2165fc0a9541b73ad9759
Reviewed-on: https://go-review.googlesource.com/23225
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>