[CoroSplit] Handle argument being the frame pointer (PR54523)

If the frame pointer is an argument of the original pointer (which
happens with opaque pointers), then we currently first replace the
argument with undef, which will prevent later replacement of the
old frame pointer with the new one.

Fix this by replacing arguments with some dummy instructions first,
and then replacing those with undef later. This gives us a chance
to replace the frame pointer before it becomes undef.

Fixes https://github.com/llvm/llvm-project/issues/54523.

Differential Revision: https://reviews.llvm.org/D122375