Set a valid reason for disabling optimization when using
%NeverOptimizeFunction.
TEST=mjsunit/compiler/regress-445732
BUG=chromium:445732
LOG=y
TBR=machenbach@chromium.org
Review URL: https://codereview.chromium.org/832003002
Cr-Commit-Position: refs/heads/master@{#25949}
