intel/compiler: Stop assuming the entrypoint is called "main"

This isn't true for Vulkan so we have to whack it to "main" in anv which
is silly.  Instead of walking the list of functions and asserting that
everything is named "main" and hoping there's only one function named
"main", just use the nir_shader_get_entrypoint() helper which has better
assertions anyway.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/compiler/brw_fs.cpp

@@ -6811,9 +6811,6 @@ fs_visitor::run_fs(bool allow_spilling, bool do_rep_send)
                  retype(dispatch_mask, BRW_REGISTER_TYPE_UW));
       }
 
-      /* Generate FS IR for main().  (the visitor only descends into
-       * functions called "main").
-       */
       emit_nir_code();
 
       if (failed)