gccgo: can shift by complex number

[dvyukov]

gccgo successfully compiles the following program:

package a
var a = 0 << complex64(1)

go/types say:

go.go:2: invalid operation: 0 << complex64(1) (shift count type complex64, must be unsigned integer)

gccgo must unlearn how to shift by complex numbers.

#11567

[dvyukov]

@paranoiacblack

[paranoiacblack]

I should note whatever changes fixes #11616, should also fix this.

[paranoiacblack]

#11616 is fixed.