This CL disables the X86 assembler tests by default. They take too long
to compile, so there's very little point in running them with the other
unittests.

This CL fixes a bug introduced in
https://codereview.chromium.org/1260163003/ that caused liveness
analysis to assert due to a uninitialized Variable.

BUG=
R=jvoung@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1266863002.