This is a follow-up to the introduction of `convert-to-llvm`: it is
supposed to be a unifying pass through the
`ConvertToLLVMPatternInterface`, but some specific conversion (like the
GPU target) aren't vanilla LLVM target. Instead they need extra
customizations that are specific to LLVM-on-GPUs and our custom runtime
wrappers.
This change make the GpuToLLVMConversionPass just as pluggable as the
`convert-to-llvm` by using the same mechanism.
9e7b6f46ba