[llvm] [NFC][AMDGPU] Add cost tests for narrow integer to fp casts (PR #225341)

[Dmitry Sidorov via llvm-commits]

MrSidims wrote:

> > @frederik-h I hope I addressed your comments
> 
> Regarding the CostModel tests, yes. This is more of a question about the whole PR stack: Could there be CodeGen tests which demonstrate the effect of the cost model change?

Kinda. There are few benchmarks from https://github.com/zjin-lcf/HeCBench as well as llama.cpp bench that are affected by the cost model change. Let me try to reduce the samples.

https://github.com/llvm/llvm-project/pull/225341