nir: add nir_shader_compiler_options::lower_to_scalar

This will replace PIPE_SHADER_CAP_SCALAR_ISA.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/compiler/brw_compiler.c

@@ -50,6 +50,7 @@
    .lower_base_vertex = true
 
 #define COMMON_SCALAR_OPTIONS                                                 \
+   .lower_to_scalar = true,                                                   \
    .lower_pack_half_2x16 = true,                                              \
    .lower_pack_snorm_2x16 = true,                                             \
    .lower_pack_snorm_4x8 = true,                                              \