For local variables of type i64 and also for i64 constants, it makes
sense to store the constant value in the {VarState} instead of loading
the value into a register immediately. This also helps with some
instructions like i64 bitshifts, but also general patterns like
incrementing an i64 local variable by a fixed number.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: Ibed15228bbc53632dd3e60d7862ff2fbcb9832ca
Reviewed-on: https://chromium-review.googlesource.com/904443
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51179}