compiler: Function_type and Backend_function_type should not be identical

Function_type and Backend_function_type have different backend
representations, so they should not be identical. Otherwise it
confuses Type::type_btypes map.

Change-Id: Iff35cd65591ccf3395a4b64ac1e46f550e4f5140
Reviewed-on: https://go-review.googlesource.com/90975
Reviewed-by: Ian Lance Taylor <iant@golang.org>
2 files changed