Add unreachable instruction to Subzero.

Also fixes error messages on instruction operands, to print
out the operand (rather than pointer to it), since we can
now print out operands.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=389
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/577703002