We cannot just blindly make a representation selection for Phi or Select
based on the truncations, but we also need to consider the type of the
inputs (or actually of the Phi/Select node itself). We can only use
Word32 representation based on Word32 truncation if the inputs are
Number or Oddball, same for Float64.

R=epertoso@chromium.org
BUG=v8:5255

Review-Url: https://codereview.chromium.org/2206553002
Cr-Commit-Position: refs/heads/master@{#38241}