intel/compiler: Add assert that set bits are within mask

We generate bitfields of bits that we want to retain (mask) and bits
that we want to set (brw_mode) in the cr0 register, so the bits we want
to set should be in the set of bits we want to retain.

Also, remove the initialization of mask from
fs_visitor::emit_shader_float_controls_execution_mode since
brw_rnd_mode_from_nir initializes the mask parameter unconditionally.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5566>