@uniforms dependent on @index values fail

I may have missed something important, but this code is currently failing on the CPU

```
using KernelAbstractions

@kernel function f_test_kernel!(input)
tid = @index(Global, Linear)
@uniform a = tid

end

function f_test!(input; numcores = 4, numthreads = 256)

if isa(input, Array)
kernel! = f_test_kernel!(CPU(), numcores)
else
kernel! = f_test_kernel!(CUDADevice(), numthreads)
end

kernel!(input, ndrange=size(input))
end

f_test!(zeros(10))

```

With `tid not defined`:

```
julia> wait(f_test!(zeros(10)))
ERROR: TaskFailedException
Stacktrace:
[1] wait
@ ./task.jl:322 [inlined]
[2] wait
@ ~/projects/KernelAbstractions.jl/src/cpu.jl:65 [inlined]
[3] wait (repeats 2 times)
@ ~/projects/KernelAbstractions.jl/src/cpu.jl:29 [inlined]
[4] top-level scope
@ REPL[3]:1
[5] top-level scope
@ ~/.julia/packages/CUDA/YpW0k/src/initialization.jl:52

nested task error: UndefVarError: tid not defined
Stacktrace:
[1] cpu_f_test_kernel!(::KernelAbstractions.CompilerMetadata{KernelAbstractions.NDIteration.DynamicSize, KernelAbstractions.NDIteration.DynamicCheck, CartesianIndex{1}, CartesianIndices{1, Tuple{Base.OneTo{Int64}}}, KernelAbstractions.NDIteration.NDRange{1, KernelAbstractions.NDIteration.DynamicSize, KernelAbstractions.NDIteration.StaticSize{(4,)}, CartesianIndices{1, Tuple{Base.OneTo{Int64}}}, Nothing}}, ::Vector{Float64})
@ ./none:0 [inlined]
[2] overdub
@ ./none:0 [inlined]
[3] __thread_run(tid::Int64, len::Int64, rem::Int64, obj::KernelAbstractions.Kernel{CPU, KernelAbstractions.NDIteration.StaticSize{(4,)}, KernelAbstractions.NDIteration.DynamicSize, typeof(cpu_f_test_kernel!)}, ndrange::Tuple{Int64}, iterspace::KernelAbstractions.NDIteration.NDRange{1, KernelAbstractions.NDIteration.DynamicSize, KernelAbstractions.NDIteration.StaticSize{(4,)}, CartesianIndices{1, Tuple{Base.OneTo{Int64}}}, Nothing}, args::Tuple{Vector{Float64}}, dynamic::KernelAbstractions.NDIteration.DynamicCheck)
@ KernelAbstractions ~/projects/KernelAbstractions.jl/src/cpu.jl:157
[4] __run(obj::KernelAbstractions.Kernel{CPU, KernelAbstractions.NDIteration.StaticSize{(4,)}, KernelAbstractions.NDIteration.DynamicSize, typeof(cpu_f_test_kernel!)}, ndrange::Tuple{Int64}, iterspace::KernelAbstractions.NDIteration.NDRange{1, KernelAbstractions.NDIteration.DynamicSize, KernelAbstractions.NDIteration.StaticSize{(4,)}, CartesianIndices{1, Tuple{Base.OneTo{Int64}}}, Nothing}, args::Tuple{Vector{Float64}}, dynamic::KernelAbstractions.NDIteration.DynamicCheck)
@ KernelAbstractions ~/projects/KernelAbstractions.jl/src/cpu.jl:130
[5] (::KernelAbstractions.var"#33#34"{Nothing, Nothing, typeof(KernelAbstractions.__run), Tuple{KernelAbstractions.Kernel{CPU, KernelAbstractions.NDIteration.StaticSize{(4,)}, KernelAbstractions.NDIteration.DynamicSize, typeof(cpu_f_test_kernel!)}, Tuple{Int64}, KernelAbstractions.NDIteration.NDRange{1, KernelAbstractions.NDIteration.DynamicSize, KernelAbstractions.NDIteration.StaticSize{(4,)}, CartesianIndices{1, Tuple{Base.OneTo{Int64}}}, Nothing}, Tuple{Vector{Float64}}, KernelAbstractions.NDIteration.DynamicCheck}})()
@ KernelAbstractions ~/projects/KernelAbstractions.jl/src/cpu.jl:22

```

In my case, I wanted to use `@uniform elem = input[tid]` as a separate element variable, which I expect to be a fairly common use-case

Start with the CPU execution path in src/cpu.jl, especially __thread_run and __run shown in the trace, and trace how @uniform handles values derived from @index(Global, Linear). Reproduce the provided f_test_kernel! example on CPU and verify that @uniform elem = input[tid] works without an UndefVarError.

Written by the indexing model from the issue text.