[mlir] Fix a warning

This patch fixes:

  mlir/lib/Dialect/Arithmetic/Transforms/ExpandOps.cpp:161:52: error:
  'static_assert' with no message is a C++17 extension
  [-Werror,-Wc++17-extensions]

mlir/lib/Dialect/Arithmetic/Transforms/ExpandOps.cpp

@@ -158,7 +158,8 @@ public:
     Location loc = op.getLoc();
     // If any operand is NaN, 'cmp' will be true (and 'select' returns 'lhs').
     static_assert(pred == arith::CmpFPredicate::UGT ||
-                  pred == arith::CmpFPredicate::ULT);
+                  pred == arith::CmpFPredicate::ULT,
+                  "pred must be either UGT or ULT");
     Value cmp = rewriter.create<arith::CmpFOp>(loc, pred, lhs, rhs);
     Value select = rewriter.create<SelectOp>(loc, cmp, lhs, rhs);