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}