fix LLVM assertion failure on VecElement of aggregate

[JeffBezanson]

I encountered this while rebasing #16622:

julia> VecElement((1,2))
julia: /home/jeff/src/julia-master/julia/deps/srccache/llvm-3.7.1/lib/IR/Instructions.cpp:1521: void llvm::InsertValueInst::init(llvm::Value*, llvm::Value*, llvm::ArrayRef<unsigned int>, const llvm::Twine&): Assertion `ExtractValueInst::getIndexedType(Agg->getType(), Idxs) == Val->getType() && "Inserted value must match indexed type!"' failed.

Now, perhaps one is not supposed to make a VecElement containing a struct-like type, but where this actually arose was during jl_compile_specializations, which decided it needed to specialize the VecElement constructor for the disturbing type VecElement{Base.##60#61{Int64, Int64}}, which appears to be a VecElement of a closure. How that type arose is a future research question...