[mlir] Fix warnings

This patch fixes:

  mlir/lib/Dialect/OpenMP/IR/OpenMPDialect.cpp:3047:16: error: unused
  variable 'ctx' [-Werror,-Wunused-variable]

  mlir/lib/Dialect/OpenMP/IR/OpenMPDialect.cpp:3171:16: error: unused
  variable 'ctx' [-Werror,-Wunused-variable]

Author: kazutakahirata

mlir/lib/Dialect/OpenMP/IR/OpenMPDialect.cpp

@@ -3044,8 +3044,7 @@ mlir::omp ::decodeCli(Value cli) {
   if (!cli)
     return {{}, nullptr, nullptr};
 
-  MLIRContext *ctx = cli.getContext();
-  assert(cli.getType() == CanonicalLoopInfoType::get(ctx) &&
+  assert(cli.getType() == CanonicalLoopInfoType::get(cli.getContext()) &&
          "Unexpected type of cli");
 
   NewCliOp create = cast<NewCliOp>(cli.getDefiningOp());
@@ -3168,8 +3167,7 @@ void NewCliOp::getAsmResultNames(OpAsmSetValueNameFn setNameFn) {
 LogicalResult NewCliOp::verify() {
   Value cli = getResult();
 
-  MLIRContext *ctx = cli.getContext();
-  assert(cli.getType() == CanonicalLoopInfoType::get(ctx) &&
+  assert(cli.getType() == CanonicalLoopInfoType::get(cli.getContext()) &&
          "Unexpected type of cli");
 
   // Check that the CLI is used in at most generator and one consumer