Codegen bug with duplicate ivar declaration and inheriting from generic instance type

[straight-shoota]

The following code crashes the compiler in codegen:

module M
  @x = ""
end

class A
  @x = ""
end

class B(T) < A
  include M
end

class C < B(UInt64)
  @y = ""
end

C.new

Module validation failed: Invalid indices for GEP pointer type!
  %3 = getelementptr inbounds %C, ptr %0, i32 0, i32 3, !dbg !8
 (Exception)

[ysbaddaden]

I'd wager the generics are confusing the compiler to believe there are 3 ivars (@x, @x and @y) while codegen will merge both @x when generating the struct aggregate.