[mlir] Fix parsing of empty complex tensors (#134322)

After #133220 we had some empty
complex literals (`tensor<0xcomplex<f32>>`) failing to parse.

This was largely due to the ambiguity between `shape.empty()` meaning
splat (`dense<1>`) or empty literal (`dense<>`). Used type's numel to
disambiguate during verification.

Author: GleasonK

mlir/lib/AsmParser/AttributeParser.cpp

@@ -566,8 +566,10 @@ DenseElementsAttr TensorLiteralParser::getAttr(SMLoc loc, ShapedType type) {
   if (ComplexType complexTy = dyn_cast<ComplexType>(eltType)) {
     eltType = complexTy.getElementType();
     isComplex = true;
-    // Complex types have 2 elements.
-    if (shape.empty() && storage.size() != 2) {
+    // Complex types have N*2 elements or complex splat.
+    // Empty shape may mean a splat or empty literal, only validate splats.
+    bool isSplat = shape.empty() && type.getNumElements() != 0;
+    if (isSplat && storage.size() != 2) {
       p.emitError(loc) << "parsed " << storage.size() << " elements, but type ("
                        << complexTy << ") expected 2 elements";
       return nullptr;

mlir/test/IR/parser.mlir

@@ -730,6 +730,10 @@ func.func @densetensorattr() -> () {
   "complex_attr"(){bar = dense<(1.000000e+00,0.000000e+00)> : tensor<complex<f32>>} : () -> ()
   // CHECK: dense<[(1.000000e+00,0.000000e+00), (2.000000e+00,2.000000e+00)]> : tensor<2xcomplex<f32>>
   "complex_attr"(){bar = dense<[(1.000000e+00,0.000000e+00), (2.000000e+00,2.000000e+00)]> : tensor<2xcomplex<f32>>} : () -> ()
+  // CHECK: dense<> : tensor<0xcomplex<i64>>
+  "complex_attr"(){bar = dense<> : tensor<0xcomplex<i64>>} : () -> ()
+  // CHECK: dense<> : tensor<2x0xcomplex<i64>>
+  "complex_attr"(){bar = dense<> : tensor<2x0xcomplex<i64>>} : () -> ()
   return
 }