Nested AddRec is already rejected by the handling in pushSCEV().

Add `dead_on_return` attribute, which is meant to be taken advantage
by the frontend, and states that the memory pointed to by the argument
is dead upon function return. As with `byval`, it is supposed to be
used for passing aggregates by value. The difference lies in the ABI:
`byval` implies that the pointer is explicitly passed as argument to
the callee (during codegen the copy is emitted as per byval contract),
whereas a `dead_on_return`-marked argument implies that the copy
already exists in the IR, is located at a specific stack offset within
the caller, and this memory will not be read further by the caller upon
callee return – or otherwise poison, if read before being written.

RFC: https://discourse.llvm.org/t/rfc-add-dead-on-return-attribute/86871.

Add support for CSE of writeonly calls, similar to the existing support
for readonly calls.

Currently predicates are allocated on the heap and tracked with an
intrusive list. Use a bump pointer allocator instead, which is more
efficient. The list is no longer needed, as we don't have to track
predicates for freeing.

The bump pointer allocator is provided as a parameter for PredicateInfo
to allow reusing the same allocator for all functions during IPSCCP.

Co-authored-by: Matt Arsenault <arsenm2@gmail.com>

For readonly/readnone calls returning void we can't CSE the return
value. However, making these participate in CSE is still useful,
because it allows DCE of calls that are not willreturn/nounwind
(something no other part of LLVM is capable of removing).

The more interesting use-case is CSE for writeonly calls (not
yet supported), but I figured this change makes sense independently.

There is no impact on compile-time.