This code moves some constants used by the target lowering, so that they
are defined (and cached) during static initialization of the target
lowering, rather than looking up everytime they are used.

This CL does this for the constant zero, and predefined helper functions.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4076
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1775253003 .