[NFC][MLIR] Fix -Wtype-limits warning

[peixin]

Summary

Fix the warning: comparison of unsigned expression in ‘>= 0’ is always
true.

mlir/include/mlir/Dialect/OpenMP/OpenMPOps.td

let extraClassDeclaration = [{
     unsigned getNumVariableOperands() {
       assert(x() && "expected 'x' operand");
       assert(v() && "expected 'v' operand");
       return 2;
     }
 
     /// The i-th variable operand passed.
     Value getVariableOperand(unsigned i) {
-      assert(0 <= i && i < 2 && "invalid index position for an operand");
+      assert(i < 2 && "invalid index position for an operand");
       return i == 0 ? x() : v();
     }
   }];
 }
 
 def AtomicWriteOp : OpenMP_Op<"atomic.write"> {
 
   let summary = "performs an atomic write";
let extraClassDeclaration = [{
     unsigned getNumVariableOperands() {
       assert(address() && "expected address operand");
       assert(value() && "expected value operand");
       return 2;
     }
 
     /// The i-th variable operand passed.
     Value getVariableOperand(unsigned i) {
-      assert(0 <= i && i < 2 && "invalid index position for an operand");
+      assert(i < 2 && "invalid index position for an operand");
       return i == 0 ? address() : value();
     }
   }];
 }
 
 def AtomicUpdateOp : OpenMP_Op<"atomic.update",
                                [SingleBlockImplicitTerminator<"YieldOp">]> {