[Flang] Use llvm_unreachable to replace assert, NFC

IIUC, the `default` case should be unreachable inferred from the context.

Reviewed By: kkwli0

Differential Revision: https://reviews.llvm.org/D154344

flang/lib/Optimizer/Builder/PPCIntrinsicCall.cpp

@@ -614,7 +614,7 @@ PPCIntrinsicLibrary::genVecCmp(mlir::Type resultType,
       break;
     }
     default:
-      assert(false && "Invalid vector operation for generator");
+      llvm_unreachable("Invalid vector operation for generator");
     }
   } else if (vecTyInfo.isFloat()) {
     mlir::Value vargs[2];
@@ -631,12 +631,12 @@ PPCIntrinsicLibrary::genVecCmp(mlir::Type resultType,
       vargs[1] = argBases[0];
       break;
     default:
-      assert(false && "Invalid vector operation for generator");
+      llvm_unreachable("Invalid vector operation for generator");
     }
     auto callOp{builder.create<fir::CallOp>(loc, funcOp, vargs)};
     res = callOp.getResult(0);
   } else
-    assert(false && "invalid vector type");
+    llvm_unreachable("invalid vector type");
 
   return res;
 }