We cannot just clear the result register optimistically, because the
register allocator might assign the same register to result and buffer.
TEST=mjsunit/compiler/regress-445858
BUG=chromium:445858
LOG=y
R=jarin@chromium.org
Review URL: https://codereview.chromium.org/828303002
Cr-Commit-Position: refs/heads/master@{#25950}
