Word8 and Word16 representation is treated like Word32 for the sake of
TurboFan's representation selection, but this was missing from the
Word64 conversions.

Bug: chromium:884933, v8:4153, v8:7881, v8:8171, v8:8178
Change-Id: If7b69cdd02b12546d87bba0643e9ee9cb35cb299
Reviewed-on: https://chromium-review.googlesource.com/1229953Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55983}