BUG: called llvm_struct_type for union of generics

[lbguilherme]

class Foo(T)
  def initialize(@x : T)
  end
end

Foo(Int32).new(0)
Foo(Int64).new(0i64)
p instance_sizeof(Foo)

causes:

BUG: called llvm_struct_type for (Foo(Int32) | Foo(Int64))
.../src/compiler/crystal/codegen/llvm_typer.cr:410:7 in 'create_llvm_struct_type'
.../src/compiler/crystal/codegen/llvm_typer.cr:308:33 in 'llvm_struct_type'
.../src/compiler/crystal/codegen/llvm_typer.cr:302:5 in 'llvm_struct_type'
.../src/compiler/crystal/codegen/codegen.cr:83:26 in 'instance_size_of'
.../src/compiler/crystal/semantic/main_visitor.cr:2476:20 in 'visit'
.../src/compiler/crystal/syntax/visitor.cr:27:12 in 'accept'
.../src/compiler/crystal/semantic/main_visitor.cr:1236:21 in 'visit'
.../src/compiler/crystal/syntax/visitor.cr:27:12 in 'accept'
.../src/compiler/crystal/syntax/ast.cr:150:27 in 'accept_children'
.../src/compiler/crystal/syntax/visitor.cr:28:11 in 'accept'
.../src/compiler/crystal/semantic/main_visitor.cr:7:7 in 'visit_main'
.../src/compiler/crystal/semantic/main_visitor.cr:5:5 in 'visit_main:process_finished_hooks:cleanup'
.../src/compiler/crystal/semantic.cr:22:7 in 'semantic'
.../src/compiler/crystal/compiler.cr:141:7 in 'compile'
.../src/compiler/crystal/command.cr:265:7 in 'compile'
.../src/compiler/crystal/command.cr:184:14 in 'run_command'
.../src/compiler/crystal/command.cr:102:7 in 'run'
.../src/compiler/crystal/command.cr:49:5 in 'run'
.../src/compiler/crystal/command.cr:48:3 in 'run'
.../src/compiler/crystal.cr:8:1 in '__crystal_main'
.../src/crystal/main.cr:11:3 in '_crystal_main'
.../src/crystal/main.cr:112:5 in 'main_user_code'
.../src/crystal/main.cr:101:7 in 'main'
.../src/crystal/main.cr:135:3 in 'main'
__libc_start_main
_start
???

I don't see anything sane that could be returned from this instance_sizeof, so this should cause an error.