Fix fwd fmuladd (rust-lang#679)

Author: wsmoses

enzyme/Enzyme/AdjointGenerator.h

@@ -3914,7 +3914,7 @@ class AdjointGenerator
         auto rule = [&](Value *dif0, Value *dif1, Value *dif2) {
           Value *dif = Builder2.CreateFAdd(Builder2.CreateFMul(op0, dif1),
                                            Builder2.CreateFMul(op1, dif0));
-          return Builder2.CreateFAdd(dif, dif0);
+          return Builder2.CreateFAdd(dif, dif2);
         };
 
         Value *dif =

enzyme/test/Enzyme/ForwardMode/fmuladd.ll

@@ -23,6 +23,6 @@ declare double @__enzyme_fwddiff(double (double, double, double)*, ...)
 ; CHECK-DAG:   %[[i0:.+]] = fmul fast double %y, %"x'"
 ; CHECK-DAG:   %[[i1:.+]] = fmul fast double %x, %"y'"
 ; CHECK-NEXT:   %2 = fadd fast double %[[i1]], %[[i0]]
-; CHECK-NEXT:   %3 = fadd fast double %2, %"x'"
+; CHECK-NEXT:   %3 = fadd fast double %2, %"z'"
 ; CHECK-NEXT:   ret double %3
 ; CHECK-NEXT: }