[llvm] Nvptx port LowerBITCAST to SelectionDAG (PR #120903)

[via llvm-commits]

GrumpyPigSkin wrote:

@RKSimon with the code as it is now, it passes the NVPTX test that was applying this lowering before. However, because the function is generalised it results in 25 test failures due to the difference in code when performing bitcasts. I presume we don't want code to change across the board? Is there a limit to the size of the types we want to shift in register, and are there any platforms we don't want to apply it to?

https://github.com/llvm/llvm-project/pull/120903