Add a bunch of lowering for I64x2 operations. This allows us to enable
most of the I64x2 tests in test-run-wasm-simd.cc.

Most of these lowering are straightforward. The load splat and load
extends need an additional operation to convert the loaded Word32 nodes
to Word64.

Bug: v8:10507
Change-Id: I6c948918c03904d2b6778223a95bb2e34b692a5e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2401954Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69827}