[cleanup][wasm] Use local variables more consistently in the streaming decoder

R=clemensh@chromium.org

Bug: v8:7109
Change-Id: I8836663f4c66fd0f8c50f2cb66ab4cb8e2712f6c
Reviewed-on: https://chromium-review.googlesource.com/803274Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49782}