[NVPTX] Add NVPTXLowerStructArgs pass

This works around the limitation that PTX does not allow .param space
loads/stores with arbitrary pointers.

If a function has a by-val struct ptr arg, say foo(%struct.x *byval %d), then
add the following instructions to the first basic block :

%temp = alloca %struct.x, align 8
%tt1 = bitcast %struct.x * %d to i8 *
%tt2 = llvm.nvvm.cvt.gen.to.param %tt2
%tempd = bitcast i8 addrspace(101) * to %struct.x addrspace(101) *
%tv = load %struct.x addrspace(101) * %tempd
store %struct.x %tv, %struct.x * %temp, align 8

The above code allocates some space in the stack and copies the incoming
struct from param space to local space. Then replace all occurences of %d
by %temp.

Fixes PR21465.