We can just use the same trick here that we use with TurboFan and load
the (signaling) NaN value out of the canonical tagged root. This
improves the loop for initializing double backing stores by hoisting the
load of the constant value out of the loop.

Bug: v8:5267
Change-Id: Idcf07c0e910ecc085a8b89225613f0a8fb50a414
Reviewed-on: https://chromium-review.googlesource.com/778979Reviewed-by: Daniel Clifford <danno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49480}