(Re-)Allow to passing non-isbits when not used in kernel

Before https://github.com/JuliaGPU/GPUCompiler.jl/commit/eec85d5096370dd4e518e99519e6bd55bd2bf053 the `check_invocation` function allowed to pass arbitrary objects as long as they were not used by the kernel. This is helpful for passing pure dispatch types `f(::Type{something)`. Currently that's only possible if those types are `Core.Compiler.isconstType`.

I tried to add it back, however since then `check_invocation` does not have access to the `entry::LLVM.Function` anymore, which was used to check if the argument is used. I think this is only generated in the `emit_llvm` call, which happens after the verification step.

I think this would be a nice feature but there are certainly always ways around it, so feel free to close if out of scope :)

Minimal example / testcase:
```julia
using Pkg
pkg"activate --temp"
pkg"add KernelAbstractions, CUDA"
using KernelAbstractions
using CUDA

abstract type AbstractAction end
struct Addition{M} <: AbstractAction
meta::M
end
struct Multiplication{M} <: AbstractAction
meta::M
end

@kernel function kernel!(::Type{T}, z, x, y) where {T<:AbstractAction}
i = @index(Global)
z[i] = apply(T, x[i], y[i])
end

@inline apply(::Type{<:Addition}, x, y) = x + y
@inline apply(::Type{<:Multiplication}, x, y) = x * y

x = CuArray(1:10)
y = CuArray(1:10)
z = CuArray(zeros(10))
kernel = kernel!(get_backend(x))

# works on concrete type
kernel(Addition{:foo}, z, x, y; ndrange=length(z))
kernel(Multiplication{:foo}, z, x, y; ndrange=length(z))

# does not work on abstract type
kernel(Addition, z, x, y; ndrange=length(z))
kernel(Multiplication, z, x, y; ndrange=length(z))
```

Start with the check_invocation entry point and trace how verification now occurs before emit_llvm, since the issue identifies the missing LLVM.Function access as the constraint. Use the minimal Julia/GPU example to reproduce abstract dispatch-type arguments and compare them with concrete types. Done means non-isbits values unused by the kernel are accepted while used values remain rejected.

Written by the indexing model from the issue text.