radeonsi: basic optimizations and cleanups for viewperf

3 cleanup commits intertwined with 2 optimizations. The switch to staging buffers for VRAM uploads is an important one.

nir_lower_bool_to_int32 causes nir_opt_if to generate invalid IR after other optimizations where phis have one argument 32-bit and another argument 1-bit. @cwabbott0 for awareness.