Trailing return yields wrong CPU code.

Created by: vchuravy

The return statement causes incorrect code.

julia> @macroexpand @kernel function f()
         return nothing
       end

yields:

    function cpu_f(; )
        $(Expr(:aliasscope))
        for var"##I#422" = (KernelAbstractions.__workitems_iterspace)()
            #= /home/vchuravy/src/KernelAbstractions/src/macros.jl:211 =#
            (KernelAbstractions.__validindex)(var"##I#422") || continue
            #= /home/vchuravy/src/KernelAbstractions/src/macros.jl:212 =#
            #= /home/vchuravy/src/KernelAbstractions/src/macros.jl:213 =#
            return nothing
        end
        $(Expr(:popaliasscope))
        return nothing
    end

Which means that we will not execute more than the first iteration. Probably we should forbid return entirely.