For speculative number comparisons with SignedSmall feedback, we always
enforce either TaggedSigned or Word32 comparisons. But this is not
really beneficial if one of the inputs is already in Float64
representation; in that case it's cheaper to just convert the other
input to a Float64.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2790833004
Cr-Commit-Position: refs/heads/master@{#44327}