This fixes undefined behavior in the implicit cast from double to float
when a double literal is passed through {fround} while declaring a local
variable.

R=jkummerow@chromium.org
TEST=mjsunit/regress/regress-crbug-976934
BUG=chromium:976934

Change-Id: I0efa2bf3f89d32c445f0b9bf719880d17fe9743c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1683999Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62469}