[llvm] [InstCombine] Turn Add into Or even when undef (PR #171556)

[Ken Matsui via llvm-commits]

ken-matsui wrote:

The IR diff report shows a small reduction in generated IR, without a meaningful regression in the report. Given the in-house regressions mentioned above, I think this is still worth restoring, especially as this patch restores a transformation that LLVM performed up through LLVM 17.

https://github.com/llvm/llvm-project/pull/171556