This makes sure that all encodings of a table index are based upon a
single implementation in the {TableIndexImmediate} class. It also fixes
one encoding that wasn't extended to support u32v yet.

R=ahaas@chromium.org
TEST=unittests/WasmOpcodeLengthTest.VariableLength

Change-Id: If24b6dc5e303d2d9e1e91cb2640c7c13eac40198
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1768375Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63408}