The {movsx_b} instruction can only take byte registers (or operands) as
source. Ensure that for i8 sign extensions to i64, the src register is
moved to a temporary byte register first, similar to the same operation
on i32 a few lines above.

R=ahaas@chromium.org

Bug: chromium:918149, v8:6600
Change-Id: I17bc942127baee57279a7fc0caac9d82bd7c6bfb
Reviewed-on: https://chromium-review.googlesource.com/c/1394555Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58542}