Due to speculative optimizations, the compiler can run into situations
where it's asked perform impossible operations, like loading a tagged
element as a float64 instead. All of this is guaranteed to be in dead
code (unless there's a bug), but leads to confusion and violates
assumptions in the compiler (that make perfect sense for code that is
not dead). So teach LoadElimination not to mix up element accesses with
incompatible representations.

BUG=chromium:719479
R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2866233002
Cr-Commit-Position: refs/heads/master@{#45185}