panfrost/midgard: Implement register spilling

panfrost/midgard: Implement register spilling

Now that we run RA in a loop, before each iteration after a failed
allocation we choose a spill node and spill it to Thread Local Storage
using st_int4/ld_int4 instructions (for spills and fills respectively).

This allows us to compile complex shaders that normally would not fit
within the 16 work register limits, although it comes at a fairly steep
performance penalty.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>