Merge CASE_I32_SIGN_EXTENSION with the identical CASE_I32_UNOP, and
rename CASE_I64_SIGN_EXTENSION to CASE_I64_UNOP. This prepares the
addition of more opcodes.

R=ahaas@chromium.org

Bug: v8:9919
Change-Id: Ie1611e2b937dffab221bfd9911e8b7f2350b9d19
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1889882Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64668}