i965/fs: add emit_shader_float_controls_execution_mode() and aux functions

i965/fs: add emit_shader_float_controls_execution_mode() and aux functions

We need this function to emit code that setups the control register
later with the defined execution mode for the shader. Therefore, we
emit it as the first instruction.

v2:
- Fix bug in setting the default mode mask in brw_rnd_mode_from_nir().
- Fix support for rounding modes in brw_rnd_mode_from_nir().

v3:
- Updated to renamed shader info member and enum values (Andres).

v4:
- Add actual emission as first instruction of emit_nir_code (Caio).

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>