[BACKEND] Remove special handling for bf16 in fp->int, int->fp handling

[davidberard98]

This PR removes some special handling for int->bf16 and bf16->int conversions in the TritonNVIDIAGPU->LLVM lowerings, in order to support, e.g. cvt.bf16.s32 and cvt.s32.bf16 instructions that are now available on Hopper.

Before this PR - there was some special handling for conversions to and from bf16; for int->bf16, the conversion would be done as a int->fp32 followed by fp32->bf16. Presumably, this was done because, before sm90, the ptx "cvt" instruction doesn't support conversions to/from bf16.

However, sm90 does support direct conversions to/from bf16; so this PR removes this special handling in order to make use of the direct cvt instructions. For Ampere, it looks like the special handling is no longer needed and llvm handles the details of different hardware implementations (perhaps thanks to llvm/llvm-project#74827?)

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