LLVMType of MtlDeviceArray needs changing/manipulation

[max-Hawkins]

Core.LLVMPtr's LLVM type is an opaque pointer that causes improper bitcasts when used in the context of MtlDeviceArrays. To avoid this, some manipulation of the pointer type is done in GPUCompiler.jl/src/metal.jl. I'm opening this issue so there can be some shared discussion/thoughts on more systemic resolutions of this problem.

Example code:

using Metal
using LLVM
convert(LLVM.LLVMType, MtlDeviceArray{Float32,1,1})
convert(LLVM.LLVMType, Core.LLVMPtr{Float32,1})