AnalyzeLoopAssignment did not take into account that 'let' shifts local
indexes.

Drive-by: Use gTest infrastructure in AnalyzeLoopAssignment tests
(EXPECT_*) instead of CHECKs.

Bug: v8:9495
Change-Id: Ic0ddb5edfde48acf172f4cac9bdcd0312b6121a0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2567955
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71595}