ShiftAdd tests lane_size * len(test_arr)^2, which is a lot of cases. In
local runs of test-run-wasm-simd, I see I32x4ShiftAdd show up
consistently as taking up a lot of time. We reduce the number of cases
tested to lane_size * len(test_arr), which stills give us very good
coverage.

Change-Id: Ida99a378f0dd9b513e8c84a2da3beebf65f0f356
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3105374
Auto-Submit: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76441}