This is a speculative fix, but I believe it might work.

Idea is to keep `start_from` as a double while establishing an appropriate
index to begin iteration. This should keep -Infinity intact rather than
converting it to a positive high value, which would break the algorithm.
This is similar to what had been implemented on the fast path before it was
changed to send non-Smis to the slow path.

BUG=v8:5986
R=bmeurer@chromium.org, cbruni@chromium.org, machenbach@chromium.org, adamk@chromium.org

Change-Id: I9965fd2e75a8972f3f1c7a18e51bd580030a66ea
Reviewed-on: https://chromium-review.googlesource.com/445857
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43358}