[clang] [llvm] [AMDGPU] Introduce a new generic target `gfx9-4-generic` (PR #115190)

[Shilei Tian via cfe-commits]

shiltian wrote:

Still need to resolve the following test failures due to mismatch check lines with gfx942.

```
  LLVM :: CodeGen/AMDGPU/div-rem-by-constant-64.ll
  LLVM :: CodeGen/AMDGPU/llvm.amdgcn.iglp.opt.exp.large.mir
  LLVM :: CodeGen/AMDGPU/llvm.amdgcn.iglp.opt.exp.small.mir
```

https://github.com/llvm/llvm-project/pull/115190