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

[zhijian lin via llvm-commits]

diggerlin 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.

I added some printf into the `DeadArgumentElimination.cpp` , it run for AIX OS,  it is IR to IR pass which is invoked by `clang`(not invoked by `llc`).

in the line https://github.com/intel/llvm/blob/sycl/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp#L336, it converts the unused argument to `poison`.

the patch https://reviews.llvm.org/D125983 change the unused argument from `undef` to `poison`

 but according to the https://releases.llvm.org/11.0.0/docs/LangRef.html#poison-values
 
 there is description:
 
 There is currently no way of representing a poison value in the IR; they only exist when produced by operations such as [add](https://releases.llvm.org/11.0.0/docs/LangRef.html#i-add) with the nsw flag.
 
 in the patch https://reviews.llvm.org/D125983, the poison can be represented a argument.  
 
 do the document need to be modified?

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