This adds support for the {i64.const} opcode. Since this makes i64
values show up on the wasm stack, quite some code paths need to handle
them. The {CheckSupportedType} method still returns false for kWasmI64,
which will be changed in a follow-up CL. That requires more changes
since it unlocks more uses of i64, e.g. in loads and stores.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ie012d0cd3db001f8693573fd16a3cfafe187009b
Reviewed-on: https://chromium-review.googlesource.com/893319
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51105}