[llvm] [NFC][VectorCombine] Add tests for narrow bitcast of shuffle (PR #143085)

[via llvm-commits]

ParkHanbum wrote:

The TODO list remaining in PhaseOrdering/X86/Shuffle-inseltpoison.ll is a test that cannot be optimized, so remove the TODO.

TODO is a test that cannot be optimized, but the intention is to leave items to be tested after Narrow or Squash is implemented, which we found worthy of leaving, so we left new ones for the VectorCombin path.

Please let me know if I did anything wrong.

https://github.com/llvm/llvm-project/pull/143085