This leads to a better handling of the Smi case when we introduce a checked truncation from a number or oddbal to a 32 bit word, which we were previously doing by concatenating a Smi to float64 conversion with a float64 to word32 truncation.

BUG=

Review-Url: https://codereview.chromium.org/2191503002
Cr-Commit-Position: refs/heads/master@{#38091}