[LLVM] remove redundant uses of dyn_cast (NFC) (#189105)
This removes dyn_cast invocations where the argument is already of the
target type (including through subtyping). This was created by adding a
static assert in dyn_cast and letting an LLM iterate until the code base
compiled. I then went through each example and cleaned it up. This does
not commit the static assert in dyn_cast, because it would prevent a lot
of uses in templated code. To prevent backsliding we should instead add
an LLVM aware version of
https://clang.llvm.org/extra/clang-tidy/checks/readability/redundant-casting.html
(or expand the existing one).
