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

[Matt Arsenault via llvm-commits]

================
@@ -6367,6 +6367,8 @@ void Verifier::visitIntrinsicCall(Intrinsic::ID ID, CallBase &Call) {
           "SGPR arguments must have the `inreg` attribute", &Call);
     Check(!Call.paramHasAttr(3, Attribute::InReg),
           "VGPR arguments must not have the `inreg` attribute", &Call);
+    Check(isa_and_present<UnreachableInst>(Call.getNextNode()),
+          "amdgcn_cs_chain must precede unreachable", &Call);
----------------
arsenm wrote:

```suggestion
          "llvm.amdgcn.cs.chain must be followed by unreachable", &Call);
```

Message consistency with similar cases 

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