i965/nir/vec4: Implement loading values from an UBO

i965/nir/vec4: Implement loading values from an UBO

Based on the vec4_visitor IR implementation for the ir_binop_load_ubo
operation. Notice that unlike the vec4_visitor IR, adding the !=0
comparison for UBO bools is not needed here because that comparison is
already added by the nir_visitor when processing the ir_binop_load_ubo
(in UBOs "true" is any value different from zero, but for us is ~0).

Adds NIR instrinsics:

   * nir_intrinsic_load_ubo_indirect
   * nir_intrinsic_load_ubo

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>