gc: handle complex CONVNOP

Fixes #2256.

R=ken2
CC=golang-dev
https://golang.org/cl/5044047
diff --git a/test/ken/cplx2.go b/test/ken/cplx2.go
index b36e93e..89f4a04 100644
--- a/test/ken/cplx2.go
+++ b/test/ken/cplx2.go
@@ -105,4 +105,16 @@
 		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")
+	}
 }