radeonsi: remove prim discard CS, reduce CPU overhead of si_update_shaders, si_emit_spi_map, etc. (big MR)

These are CPU overhead improvements motivated by viewperf/snx.

Changes:

• shader variant keys are added into si_context and they are updated in bind & set functions instead of si_shader_selector_key, reducing the overhead of si_update_shaders
• si_update_shaders is cleaned up and moved to become a C++ template in si_state_draw.cpp to reduce overhead
• si_update_spi_map is cleaned up; its loops, memcmp, and memcpy are unrolled using 33 C++ template instantiations to reduce overhead
• the primitive discard compute shader is removed
• the NGG passthrough mode is removed
• SPI_SHADER_PGM_HI_* registers are no longer set for every shader
• etc.

Tested on gfx10 (Navi10) and gfx8 (Polaris11).