[mlir] Add folder for complex.ReOp and complex.ImOp.

[akuegel]

Summary

Now that complex constants are supported, we can also fold.

[akuegel]

Sorry, I was careless and didn't test the other tests apart from the ones I added in this patch.
I reverted this patch, and will look into why some other tests are failing.

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

let description = [{
     %a = complex.im %b : complex<f32>
     ```
   }];
 
   let arguments = (ins Complex<AnyFloat>:$complex);
   let results = (outs AnyFloat:$imaginary);
 
   let assemblyFormat = "$complex attr-dict `:` type($complex)";
+  let hasFolder = 1;
 }
 
 //===----------------------------------------------------------------------===//
 // MulOp
 //===----------------------------------------------------------------------===//
 
 def MulOp : ComplexArithmeticOp<"mul"> {
   let summary = "complex multiplication";
let description = [{
     %a = complex.re %b : complex<f32>
     ```
   }];
 
   let arguments = (ins Complex<AnyFloat>:$complex);
   let results = (outs AnyFloat:$real);
 
   let assemblyFormat = "$complex attr-dict `:` type($complex)";
+  let hasFolder = 1;
 }
 
 
 //===----------------------------------------------------------------------===//
 // SubOp
 //===----------------------------------------------------------------------===//
 
 def SubOp : ComplexArithmeticOp<"sub"> {

mlir/lib/Dialect/Complex/IR/ComplexOps.cpp

 using namespace mlir::complex;
 
 //===----------------------------------------------------------------------===//
 // TableGen'd op method definitions
 //===----------------------------------------------------------------------===//
 
 #define GET_OP_CLASSES
 #include "mlir/Dialect/Complex/IR/ComplexOps.cpp.inc"
+
+OpFoldResult ReOp::fold(ArrayRef<Attribute> operands) {
+  ArrayAttr arrayAttr = operands[0].dyn_cast<ArrayAttr>();
+  return arrayAttr[0];
+}
+
+OpFoldResult ImOp::fold(ArrayRef<Attribute> operands) {
+  ArrayAttr arrayAttr = operands[0].dyn_cast<ArrayAttr>();
+  return arrayAttr[1];
+}

mlir/test/Dialect/Complex/canonicalize.mlir

+// RUN: mlir-opt %s -canonicalize | FileCheck %s
+
+// CHECK-LABEL: func @real_of_const(
+func @real_of_const() -> f32 {
+  // CHECK: %[[CST:.*]] = constant 1.000000e+00 : f32
+  // CHECK-NEXT: return %[[CST]] : f32
+  %complex = constant [1.0 : f32, 0.0 : f32] : complex<f32>
+  %1 = complex.re %complex : complex<f32>
+  return %1 : f32
+}
+
+// CHECK-LABEL: func @imag_of_const(
+func @imag_of_const() -> f32 {
+  // CHECK: %[[CST:.*]] = constant 0.000000e+00 : f32
+  // CHECK-NEXT: return %[[CST]] : f32
+  %complex = constant [1.0 : f32, 0.0 : f32] : complex<f32>
+  %1 = complex.im %complex : complex<f32>
+  return %1 : f32
+}