iris: Make room for additional state dirty bits.
Iris is running out of dirty bits in its context data structure, which is a bit of a problem for implementing new functionality. E.g. MR !5018 no longer applies cleanly.
This branch addresses the problem by splitting off the stage-specific dirty bits into a separate "stage_dirty" mask, which effectively extends the number of available dirty bits to 128.
Edited by Francisco Jerez