nv50/ir/nir: implement variable indexing

nv50/ir/nir: implement variable indexing

We store those arrays in local memory and reserve some space for each of the
arrays. With NIR we could store those arrays packed, but we don't do that yet
as it causes MemoryOpt to generate unaligned memory accesses.

v3: use fixed size vec4 arrays until we fix MemoryOpt
v4: fix for 64 bit types
v5: use loadFrom helper
v8: don't require C++11 features
v9: convert to C++ style comments

Signed-off-by: Karol Herbst <kherbst@redhat.com>