agxv: Lots of fixes for dEQP-VK.spirv_assembly.*
Most of these commits (everything up to and including "agx: Add AGX_MESA_DEBUG=spill option") are from my register spilling branch. Together they implement register spilling in the compiler (so far limited to select compute kernels, but good enough to get the widephi
test passing).
The following commits fix assorted tests in the dEQP-VK.spirv_assembly.*
group, which I've been working through. Lot of goodies in here.