The valid store types of a {Float64Array} heap view are specified to be
"float?" and "double?". We correctly accepted both types but forgot to
emit the appropriate conversion in the "float?" case. This just adds the
missing conversion expression.

R=clemensh@chromium.org
TEST=mjsunit/regress/regress-crbug-898974
BUG=chromium:898974,v8:8347

Change-Id: I306b10e2088185b1522da29b1a113908ef9925f2
Reviewed-on: https://chromium-review.googlesource.com/c/1301499
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57025}