Changes:
- Move enhancement of locals_count by 1 inside AnalyzeLoopAssignment.
- Update documentation of AnalyzeLoopAssignment.
- Factor out invocation to OpcodeLength();
- Use uint32_t for locals count consistently in related testing
  functions.

Change-Id: I5bb5a324c4f4ed1aafc37849f3762d7a9630da51
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2549966Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71312}