Use NewPM with extension points to enable back-end passes

https://reviews.llvm.org/D148561 should make it possible to use PMBuilder extension points with custom pipelines. Once we have our own NewPM pipeline (which depends on https://github.com/maleadt/LLVM.jl/issues/344) we should make use of this feature in order to enable back-end specific passes, like NVPTX's NVVMReflect, or the many AMD-specific passes in the AMDGPU back-end.

Ref https://github.com/JuliaGPU/GPUCompiler.jl/issues/263

Review LLVM review D148561 and the dependency in LLVM.jl issue 344 to understand the NewPM extension-point support and custom pipeline status. Then compare the needs described in GPUCompiler.jl issue 263, including NVPTX's NVVMReflect and AMDGPU-specific passes; done means the project’s NewPM pipeline enables those backend-specific passes.

Written by the indexing model from the issue text.