[llvm] [NFC][PowerpC] Add testcases for locking down the xxeval instruction support for ternary operators (PR #141601)

[Lei Huang via llvm-commits]

https://github.com/lei137 commented:

For easy future maintenance, it's best if we generate the checks via `update_llc_test_checks.py`

https://github.com/llvm/llvm-project/pull/141601