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

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

I'm not clear on what you're doing - you mention InstCombine but add VectorCombine tests, plus won't the PhaseOrdering tests still update if you do improve VectorCombine handling so shouldn't the TODOs stay until then?

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