[llvm] [AMDGPU][SIPreEmitPeephole] mustRetainExeczBranch: use BranchProbability and TargetSchedmodel (PR #109818)

[Juan Manuel Martinez Caamaño via llvm-commits]

https://github.com/jmmartinez commented:

In a future PR this code should fall through if the optimized branch is an execz:

```cpp
      case AMDGPU::S_CBRANCH_VCCZ:
      case AMDGPU::S_CBRANCH_VCCNZ:
        Changed |= optimizeVccBranch(MI);
        break;
```

https://github.com/llvm/llvm-project/pull/109818