Ensure we only lower SpeculativeNumberBinops to a pure operator for
non-string plain primitives. Previously we could lower if a value might be
the-hole, however this would fail a CHECK in ConvertInputsToNumber which
expects a plain primitive.

BUG=chromium:772420

Change-Id: I0c755d10db7afd9cabfb638eca5662d70dfc8d51
Reviewed-on: https://chromium-review.googlesource.com/715717
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48649}