SimpleVector lacks appropriate `isdefined` call

[timholy]

julia> type MyType{T,N} end

julia> MyType{Int,2}
MyType{Int64,2}

julia> isdefined(MyType.name.cache, 0)
false

julia> isdefined(MyType.name.cache, 1)
true

julia> isdefined(MyType.name.cache, 2)
false

julia> MyType{Float32, 5}
MyType{Float32,5}

julia> isdefined(MyType.name.cache, 2)
false

julia> MyType.name.cache
svec(MyType{Float32,5},MyType{Int64,2},Error showing value of type SimpleVector:
ERROR: UndefRefError: access to undefined reference
 in show_delim_array at show.jl:189
 in show at show.jl:213
 in anonymous at show.jl:1232
 in with_output_limit at ./show.jl:1209
 in showlimited at show.jl:1231
 in writemime at replutil.jl:4
 in display at REPL.jl:113
 in display at REPL.jl:116
 in display at multimedia.jl:151
 in print_response at REPL.jl:133
 in print_response at REPL.jl:120
 in anonymous at REPL.jl:599
 in run_interface at ./LineEdit.jl:1554
 in run_frontend at ./REPL.jl:843
 in run_repl at ./REPL.jl:166
 in _start at ./client.jl:451

[timholy]

I should add that a fix is coming, but I have other immediate obligations.

[carnaval]

(note: this builtin could probably use a specialized codegen since it's a bit wasteful to go through the jlcall thing when all we want to do is a null ptr check)

[timholy]

Good point. Since I really haven't dug into LLVM*, implementing that is currently above my pay grade, but I agree with the idea.

*yet---it seems that eventually one gets tempted into every nook and cranny