[mlir][complex] Minor fixes in ComplexToStandard test cases.

[ingomueller-net]

Summary

One FileCheck line was hard-coding variable names. Another line
mis-indented the second line of a function header.

[akuegel]

Thank you :)

mlir/test/Conversion/ComplexToStandard/convert-to-standard.mlir

 func.func @complex_sign(%arg: complex<f32>) -> complex<f32> {
   %sign = complex.sign %arg: complex<f32>
   return %sign : complex<f32>
 }
 // CHECK: %[[REAL:.*]] = complex.re %[[ARG]] : complex<f32>
 // CHECK: %[[IMAG:.*]] = complex.im %[[ARG]] : complex<f32>
 // CHECK: %[[ZERO:.*]] = arith.constant 0.000000e+00 : f32
 // CHECK: %[[REAL_IS_ZERO:.*]] = arith.cmpf oeq, %[[REAL]], %[[ZERO]] : f32
-// CHECK: %[[IMAG_IS_ZERO:.*]] = arith.cmpf oeq, %1, %cst : f32
+// CHECK: %[[IMAG_IS_ZERO:.*]] = arith.cmpf oeq, %[[IMAG]], %[[ZERO]] : f32
 // CHECK: %[[IS_ZERO:.*]] = arith.andi %[[REAL_IS_ZERO]], %[[IMAG_IS_ZERO]] : i1
 // CHECK: %[[REAL2:.*]] = complex.re %[[ARG]] : complex<f32>
 // CHECK: %[[IMAG2:.*]] = complex.im %[[ARG]] : complex<f32>
 // CHECK: %[[SQR_REAL:.*]] = arith.mulf %[[REAL2]], %[[REAL2]] : f32
 // CHECK: %[[SQR_IMAG:.*]] = arith.mulf %[[IMAG2]], %[[IMAG2]] : f32
 // CHECK: %[[SQ_NORM:.*]] = arith.addf %[[SQR_REAL]], %[[SQR_IMAG]] : f32
 // CHECK: %[[NORM:.*]] = math.sqrt %[[SQ_NORM]] : f32
 // CHECK: %[[REAL_SIGN:.*]] = arith.divf %[[REAL]], %[[NORM]] : f32

 // CHECK: %[[NEG_IMAG:.*]] = arith.negf %[[IMAG]] : f32
 // CHECK: %[[RESULT:.*]] = complex.create %[[REAL]], %[[NEG_IMAG]] : complex<f32>
 // CHECK: return %[[RESULT]] : complex<f32>
 
 // -----
 
 // CHECK-LABEL:   func.func @complex_pow
 func.func @complex_pow(%lhs: complex<f32>,
-                         %rhs: complex<f32>) -> complex<f32> {
   %pow = complex.pow %lhs, %rhs : complex<f32>
   return %pow : complex<f32>
 }
 
 // -----
 
 // CHECK-LABEL:   func.func @complex_rsqrt
 func.func @complex_rsqrt(%arg: complex<f32>) -> complex<f32> {