[mlir] Add `complex.atan2` operation.

[bkramer]

looks good

mlir/include/mlir/Dialect/Complex/IR/ComplexOps.td

let description = [{
 
     ```mlir
     %a = complex.add %b, %c : complex<f32>
     ```
   }];
 }
 
 //===----------------------------------------------------------------------===//
+// Atan2
+//===----------------------------------------------------------------------===//
+
+def Atan2Op : ComplexArithmeticOp<"atan2"> {
+  let summary = "complex 2-argument arctangent";
+  let description = [{
+    For complex numbers it is expressed using complex logarithm
+    atan2(y, x) = -i * log((x + i * y) / sqrt(x**2 + y**2))
+
+    Example:
+
+    ```mlir
+    %a = complex.atan2 %b, %c : complex<f32>
+    ```
+  }];
+}
+
+//===----------------------------------------------------------------------===//
 // ConstantOp
 //===----------------------------------------------------------------------===//
 
 def ConstantOp : Complex_Op<"constant", [
     ConstantLike, NoSideEffect,
     DeclareOpInterfaceMethods<OpAsmOpInterface, ["getAsmResultNames"]>
   ]> {
   let summary = "complex number constant operation";

mlir/test/Dialect/Complex/ops.mlir

func.func @ops(%f: f32) {
   %pow = complex.pow %complex, %complex : complex<f32>
 
   // CHECK: complex.sqrt %[[C]] : complex<f32>
   %sqrt = complex.sqrt %complex : complex<f32>
 
   // CHECK: complex.rsqrt %[[C]] : complex<f32>
   %rsqrt = complex.rsqrt %complex : complex<f32>
 
+  // CHECK: complex.atan2 %[[C]], %[[C]] : complex<f32>
+  %atan2 = complex.atan2 %complex, %complex : complex<f32>
+
   return
 }