intel/compiler: Move idom tree calculation and related logic into analysis object

This only does half of the work.  The actual representation of the
idom tree is left untouched, but the computation algorithm is moved
into a separate analysis result class wrapped in a BRW_ANALYSIS
object, along with the intersect() and dump_domtree() auxiliary
functions in order to keep things tidy.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4012>

src/intel/compiler/brw_shader.cpp

@@ -703,7 +703,7 @@ backend_shader::backend_shader(const struct brw_compiler *compiler,
      nir(shader),
      stage_prog_data(stage_prog_data),
      mem_ctx(mem_ctx),
-     cfg(NULL),
+     cfg(NULL), idom_analysis(this),
      stage(shader->info.stage)
 {
    debug_enabled = INTEL_DEBUG & intel_debug_flag_for_shader_stage(stage);
@@ -1248,6 +1248,7 @@ backend_shader::calculate_cfg()
 void
 backend_shader::invalidate_analysis(brw::analysis_dependency_class c)
 {
+   idom_analysis.invalidate(c);
 }
 
 extern "C" const unsigned *