glsl+gallium: replace PIPE_SHADER_CAP_GLSL_16BIT_TEMPS
Previously the 16b-temps cap was controlling precision lowering for temps and constants. For all drivers we want to lower the former. But some drivers we don't want to lower the latter. So split out a different compiler option, and replace PIPE_SHADER_CAP_GLSL_16BIT_TEMPS with PIPE_SHADER_CAP_GLSL_16BIT_CONSTS
The first couple patches are just getting the ir3 cmdline compiler working again, which was useful for testing.