[llvm] [AMDGPU] add tests for loop definition of bitconvert (PR #133052)

[via llvm-commits]

Shoreshen wrote:

> > Also what are these lit tests for ? why do we need to test all widths?
> 
> Every operation with every type should be tested.
> 
> For this case, we already should have test coverage so I'm not sure I understand what is new here. We should perhaps split / replace the existing test with a more consistent pattern, but this patch is purely additive?

Hi @arsenm , I removed the old tests and split them into different files named by bit width. 

https://github.com/llvm/llvm-project/pull/133052