V8's MemMove and libc's memset do not specify how the memory is filled,
but wasm requires the direction to be backward only when the regions are
overlapping and the source address is less than the destination address.

Bug: v8:8950
Change-Id: If392967b51b33ad65fe145660f44b857559e34b1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1507316
Commit-Queue: Ben Smith <binji@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60106}