Add adjustPassManager to run target specific passes

[vchuravy]

Both opt and clang call TM->adjustPassManager(PMB) to add the target specific passes,
like NVVMReflect to the pass list.

[maleadt]

Right, that's the functionality I talked about. The problem is that this function doesn't take a pass manager, but a pass manager builder, which we don't use in GPUCompiler. The implementations of adjustPassManager register extension points, which is currently unusable to us (until we reimplement Julia's optimization pipeline as a PMB).

[jpsamaroo]

If we just wanted to feed in a Module and get access to target-specific passes after standard Julia optimizations, would this work? I'm thinking we could run it pretty close to ASM emission, since for AMDGPU, there are a ton of optimizations we miss out on because they're only exposed through this interface. We'd probably want to have simplified as many Julia-isms as possible before running those target-specific passes anyway, under the assumption that our unoptimized IR looks pretty different from what C++ would generate.

[maleadt]

I don't think so, as many passes are intended to be run at specific points during optimization, e.g., https://github.com/JuliaGPU/llvm-metal/blob/355123f9415581fa1f5fabb0f7e7227bfbc9f43d/llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp#L584-L596. Running those close to ASM emission probably isn't going to result in the behavior you want.