[llvm] [NFC][AMDGPU] Add D16 test for multiple fptrunc image sample (PR #141771)

[Harrison Hao via llvm-commits]

harrisonGPU wrote:

> What does "D16" mean?

`D16` stands for "Destination 16-bit". When enabled, `image sample` instructions return 16-bit values directly, which reduces VGPR usage and skips the need for `v_cvt_f16_f32_e32`.

https://github.com/llvm/llvm-project/pull/141771