Avoid the zero-extensions required for int32 based addressing on 64-bit
architectures by restructuring the initialization loops to work on
words. This recovers a bit of the regression on the Kraken audio-fft and
audio-beat-detection benchmarks that was introduced by the initial CL.

Bug: chromium:772669, v8:6399, v8:6901
Change-Id: I4753c254be89f2bcc7b0ea5073e469e3507408bd
Reviewed-on: https://chromium-review.googlesource.com/707098Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48377}