[llvm] [AMDGPU] Enhance verification of amdgcn.cs.chain intrinsic (PR #128162)

[Matt Arsenault via llvm-commits]

================
@@ -32,6 +32,13 @@ define amdgpu_cs_chain void @bad_exec(ptr %fn, i32 %exec, <4 x i32> inreg %sgpr,
   unreachable
 }
 
+define amdgpu_cs_chain void @not_unreachable(ptr %fn, i32 %exec, <4 x i32> inreg %sgpr, { ptr, <3 x i32> } %vgpr) {
+  ; CHECK: amdgcn_cs_chain must precede unreachable
----------------
arsenm wrote:

```suggestion
  ; CHECK: llvm.amdgcn.cs.chain must be followed by unreachable
```

https://github.com/llvm/llvm-project/pull/128162