There was a bug in spilling i64 constants, in that the half stack slot
*above* should have been filled with the high word instead of the one *below*.

Instead of just fixing this, this CL optimizes spilling x64 constants to the
stack by emitting shorter and faster code, especially if the constant fits in
31 bits (which is the majority of cases).

R=titzer@chromium.org

Bug: v8:7565,v8:6600
Change-Id: Id75ddafe82615930a84333a0c49bd515ccbcc093
Reviewed-on: https://chromium-review.googlesource.com/965062Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51985}