Unnecessary gc root slots in generated code

[Keno]

type foo
    a::Vector{Int64}
end

f(x::foo) = (a = x.a; for i = 1:100; @inbounds a[i] += 1; end)

code_llvm(f, Tuple{foo})

This generates 3 roots. We technically don't need any because there's no safepoints. However, I'd settle for 1 root for now.

[yuyichao]

Close as dup of #15369