tree-optimization/104373 - early diagnostic on unreachable code

tree-optimization/104373 - early diagnostic on unreachable code

The following improves early uninit diagnostics by computing edge
reachability using VN and ignoring unreachable blocks when looking
for uninitialized uses.  To not ICE with -fdump-tree-all the
early uninit pass needs a dumpfile since VN tries to dump statistics.

2022-02-04  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104373
	* tree-ssa-sccvn.h (do_rpo_vn): New export exposing the
	walk kind.
	* tree-ssa-sccvn.cc (do_rpo_vn): Export, get the default
	walk kind as argument.
	(run_rpo_vn): Adjust.
	(pass_fre::execute): Likewise.
	* tree-ssa-uninit.cc (warn_uninitialized_vars): Skip
	blocks not reachable.
	(execute_late_warn_uninitialized): Mark all edges as
	executable.
	(execute_early_warn_uninitialized): Use VN to compute
	executable edges.
	(pass_data_early_warn_uninitialized): Enable a dump file,
	change dump name to warn_uninit.

	* g++.dg/warn/Wuninitialized-32.C: New testcase.
	* gcc.dg/uninit-pr20644-O0.c: Remove XFAIL.