Don't disallow Bottom returns.

[maleadt]

Trial balloon. Currently, we eagerly disallow returns, even of Union{} (which indicates an error):

julia> @cuda threads=(10, 10) kernel(array_dev)
ERROR: GPU compilation of MethodInstance for kernel(::CuDeviceVector{Int64, 1, Int32}) failed
KernelError: kernel returns a value of type `Union{}`

Make sure your kernel function ends in `return`, `return nothing` or `nothing`.
If the returned value is of type `Union{}`, your Julia code probably throws an exception.
Inspect the code with `@device_code_warntype` for more details.

Stacktrace:
  [1] check_method(job::GPUCompiler.CompilerJob)
    @ GPUCompiler ~/Julia/pkg/GPUCompiler/src/validation.jl:36
  [2] macro expansion
    @ GPUCompiler ~/Julia/pkg/GPUCompiler/src/driver.jl:122 [inlined]

I'm not sure that's very user-friendly however, as it requires the user to use Cthulhu or similar to figure out where the error was introduced. If we just allow the error, it's likely that we'll be able to report something better later on:

julia> @cuda threads=(10, 10) kernel(array_dev)
ERROR: InvalidIRError: compiling MethodInstance for kernel(::CuDeviceVector{Int64, 1, Int32}) resulted in invalid LLVM IR
...
Reason: unsupported use of an undefined name (use of 'i32')
Stacktrace:
 [1] kernel
   @ ./REPL[3]:2

cc @vchuravy

[vchuravy]

I like this! Gives a much better error in the end/

[maleadt]

Yeah this seems strictly better. Because of how test suites rely on the KernelError though, we'll have to make this a breaking change (albeit one easy to adapt to).