The source register of {movsx_b} must be a byte register.

Drive-by: Add missing sign extension opcodes to wasm-constants.js.

R=herhut@chromium.org

Bug: chromium:916869
Change-Id: I571c1ea2a0e197afefc810f306eed238250cd5e0
Reviewed-on: https://chromium-review.googlesource.com/c/1386110Reviewed-by: Stephan Herhut <herhut@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58394}