We would like DecodeLocals to allow inserting new locals
in any position. This is useful for the upcoming 'let' instruction.

Bug: v8:7748
Change-Id: Ic7f2a7fba0f69ee76b0ace46bb0cecee9d047306
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2208859
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67936}