pan/bi: Schedule around blend shader register clobbering

By software ABI, a blend shader is permitted to clobber registers R0-R15. The scheduler needs to be aware of this, to avoid moving a write to one of these registers past the BLEND itself. Otherwise the schedule is invalid.

This bug affects GLES3.0, but is rare enough in practice that we had missed it. It requires a fragment shader to write to multiple render targets with attached blend shaders, and have temporaries register allocated to R0-R15 that are not read by the blend shader, but are sunk past the BLEND instruction by the scheduler. Prevents a regression when switching boolean representations on:

dEQP-GLES31.functional.shaders.builtin_functions.integer.uaddcarry.uvec4_lowp_fragment

Signed-off-by: Alyssa Rosenzweig alyssa@collabora.com Cc: mesa-stable