[llvm] using UNDEF SDNode instead of Constant SDNode when extend the UNDEF SDNode (PR #122741)

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

The issue that the effect of `DeadArgumentEliminationPass` is not fully realized is shared with other platforms that require width extension of argument values such as RISC-V and LoongArch: https://godbolt.org/z/x55oqhT8h.

https://github.com/llvm/llvm-project/pull/122741