[Flang] Use llvm_unreachable to replace assert, NFC

[lkail]

Summary

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

[kkwli0]

LTGM. Thanks

flang/lib/Optimizer/Builder/PPCIntrinsicCall.cpp

case VecOp::Cmple: {
       auto vargs2{
           convertVecArgs(builder, loc, vecTyInfo, mlir::ValueRange{callRes})};
       auto xorRes{builder.create<mlir::arith::XOrIOp>(loc, vargs2[0], vNegOne)};
 
       res = builder.createConvert(loc, returnType, xorRes);
       break;
     }
     default:
-      assert(false && "Invalid vector operation for generator");
+      llvm_unreachable("Invalid vector operation for generator");
     }
   } else if (vecTyInfo.isFloat()) {
     mlir::Value vargs[2];
     switch (vop) {
     case VecOp::Cmpge:
     case VecOp::Cmpgt:
       vargs[0] = argBases[0];
       vargs[1] = argBases[1];
       break;
     case VecOp::Cmple:
     case VecOp::Cmplt:
       // Swap the arguments as xvcmpg[et] is used
       vargs[0] = argBases[1];
       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;
 }
 
 } // namespace fir