With this change, we pass the select crosstest.

Since this would have introduced a three-argument version of
scalarizeInstruction, I decided to generalize it using templates.

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

Review URL: https://codereview.chromium.org/1683243003 .