Fixes bytecode mismatch between lazy and non-lazy where "this" was
marked as maybe assigned in constructors that called the super
constructor. Since this will return the hole in cases where it was not
yet initialized by super (and the hole is explicitly handled by
JSContextSpecialization::ReduceJSLoadContext), it's safe to treat it as
a constant in all cases. In the case of lazy compilation case, "this"
is never added to the ScopeInfo so is never seen as mutable.

Bug: chromium:994719
Change-Id: I43478fbc626b19eb1533aa9dec61b7f276ae140b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1762025
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63283}