When replacing a LoadElement with variable index with its known fields,
only do it if the types match, otherwise we end up with a graph that
representation selection cannot handle. That can only happen in dead
code, but TurboFan would nevertheless crash in representation selection.

Bug: chromium:893982, chromium:899524, v8:5267, v8:6200
Change-Id: I01e645d5e01bffb911d216d37d923792d9d0beab
Reviewed-on: https://chromium-review.googlesource.com/c/1303721Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57059}