This PR replaces some instances of `undef` with `function argument
value` or `poison` or `concrete values` in several tests under
`llvm/test/Transforms/` directory. These changes align with modern LLVM
standards for better-defined behavior and test determinism. If this
small PR is okay and gets merged, I will work on the rest.
This is inspired by [this
project](https://discourse.llvm.org/t/gsoc-2024-remove-undefined-behavior-from-tests/77236/29),
work done on this by @leewei05
d80eb928c7