[llvm] AMDGPU: Implement llvm.set.rounding (PR #88587)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> @arsenm This is failing on https://lab.llvm.org/buildbot/#/builders/124/builds/10565:
> 
> ```
> /home/buildbot/as-worker-91/clang-with-lto-ubuntu/llvm-project/llvm/lib/Target/AMDGPU/SIModeRegisterDefaults.h: In function ‘constexpr uint32_t llvm::AMDGPU::decodeFltRoundToHWConversionTable(uint32_t)’:
> /home/buildbot/as-worker-91/clang-with-lto-ubuntu/llvm-project/llvm/lib/Target/AMDGPU/SIModeRegisterDefaults.h:157:0: error: the value of ‘llvm::AMDGPU::FltRoundToHWConversionTable’ is not usable in a constant expression
> /home/buildbot/as-worker-91/clang-with-lto-ubuntu/llvm-project/llvm/lib/Target/AMDGPU/SIModeRegisterDefaults.h:149:0: note: ‘llvm::AMDGPU::FltRoundToHWConversionTable’ was not initialized with a constant expression
> ```

Hopefully c7ad12d0d7606b0b9fb531b0b273bdc5f1490ddb fixes it 

https://github.com/llvm/llvm-project/pull/88587