[SDAGBuilder] Handle multi-part arguments in argument copy elision (PR63430)

When eliding an argument copy, we need to update the chain to ensure
the argument reads are performed before later writes. However, the
code doing this only handled this for the first part of the argument.
If the argument had multiple parts, the chains of the later parts were
dropped. Make sure we preserve all chains.

Fixes https://github.com/llvm/llvm-project/issues/63430.