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

[Frederik Harwath via llvm-commits]

frederik-h 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?

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