hwasan: Move fixed shadow behind opaque no-op cast as well.

This is a workaround for poor heuristics in the backend where we can
end up materializing the constant multiple times. This is particularly
bad when using outlined checks because we materialize it for every call
(because the backend considers it trivial to materialize).

As a result the field containing the shadow base value will always
be set so simplify the code taking that into account.

Differential Revision: https://reviews.llvm.org/D90425