Only print warning under MLIR_ENABLE_DIAGNOSTICS (#2943)

As explained in
https://github.com/intel/intel-xpu-backend-for-triton/pull/1048/files#r1871965306,
we would like to be consistent with upstream to only print warning under
env var `MLIR_ENABLE_DIAGNOSTICS`.

Signed-off-by: Whitney Tsang <[email protected]>

Author: whitneywhtsang

python/src/ir.cc

@@ -1680,11 +1680,11 @@ void init_triton_ir(py::module &&m) {
              if (haveDiagnostics) {
                context->printOpOnDiagnostic(true);
                context->printStackTraceOnDiagnostic(true);
+               context->getDiagEngine().registerHandler([](Diagnostic &diag) {
+                 llvm::outs() << diag << "\n";
+                 return success();
+               });
              }
-             context->getDiagEngine().registerHandler([](Diagnostic &diag) {
-               llvm::outs() << diag << "\n";
-               return success();
-             });
              if (haveDump) {
                auto printingFlags = OpPrintingFlags();
                printingFlags.elideLargeElementsAttrs(16);