gc: handle complex CONVNOP

Fixes #2256.

R=ken2
CC=golang-dev
https://golang.org/cl/5044047

src/cmd/gc/cplx.c

@@ -133,6 +133,9 @@ complexgen(Node *n, Node *res)
 		dump("\ncomplexgen-n", n);
 		dump("complexgen-res", res);
 	}
+	
+	while(n->op == OCONVNOP)
+		n = n->left;
 
 	// pick off float/complex opcodes
 	switch(n->op) {

test/ken/cplx2.go

@@ -105,4 +105,16 @@ func main() {
 		println("opcode x", ce, Ce)
 		panic("fail")
 	}
+	
+	r32 := real(complex64(ce))
+	if r32 != float32(real(Ce)) {
+		println("real(complex64(ce))", r32, real(Ce))
+		panic("fail")
+	}
+	
+	r64 := real(complex128(ce))
+	if r64 != real(Ce) {
+		println("real(complex128(ce))", r64, real(Ce))
+		panic("fail")
+	}
 }