nir + r600/nir: some new opcodes, genralized optimizations, and cleanups in the driver code

The MR adds a few new opcodes:

• four opcodes for fused compare + csel
• a find_msb_rev to account for hardware that returns the MSB counted from the leftmost bit
• compiler options + algebraic lowering/optimizations to use these opcodes where supported
• r600 specific opcodes for sin and cos

The reminder of the MR contains code to do some code clean-up and replace lowering done in the driver by defining some driver specific optimizations.