gccgo: internal compiler error in do_flatten

[dvyukov]

gccgo crashes on the following input:

package a
var e = *<--chan*e

go1: internal compiler error: in do_flatten, at go/gofrontend/expressions.cc:3652
0x5e054f Unary_expression::do_flatten(Gogo*, Named_object*, Statement_inserter*)
    ../../gcc/go/gofrontend/expressions.cc:3652
0x610a13 Expression::flatten(Gogo*, Named_object*, Statement_inserter*)
    ../../gcc/go/gofrontend/expressions.h:803
0x610a13 Flatten::expression(Expression**)
    ../../gcc/go/gofrontend/gogo.cc:3866
0x6126a6 Gogo::flatten_expression(Named_object*, Statement_inserter*, Expression**)
    ../../gcc/go/gofrontend/gogo.cc:3893
0x61fec0 Variable::flatten_init_expression(Gogo*, Named_object*, Statement_inserter*)
    ../../gcc/go/gofrontend/gogo.cc:6020
0x61ff9b Flatten::variable(Named_object*)
    ../../gcc/go/gofrontend/gogo.cc:3782
0x6169e9 Bindings::traverse(Traverse*, bool)
    ../../gcc/go/gofrontend/gogo.cc:7493
0x616b71 Gogo::traverse(Traverse*)
    ../../gcc/go/gofrontend/gogo.cc:2491
0x616dcf Gogo::flatten()
    ../../gcc/go/gofrontend/gogo.cc:3900
0x6105db go_parse_input_files(char const**, unsigned int, bool, bool)
    ../../gcc/go/gofrontend/go.cc:139
0x60c32f go_langhook_parse_file
    ../../gcc/go/go-lang.c:304

gcc version 6.0.0 2015070 (experimental) (GCC)

[dvyukov]

@paranoiacblack

[gopherbot]

CL https://golang.org/cl/12044 mentions this issue.