debug/105158 - improve debug stmt retaining for forwarder removal

debug/105158 - improve debug stmt retaining for forwarder removal

Currently when we cannot move debug stmt from a forwarder to the
destination block we drop/reset them.  But in some cases as for
the testcase we can move them to the predecessor when that has
a single successor and we can insert after the last stmt of the
block.  That allows us to preserve debug info here.

2022-04-05  Richard Biener  <rguenther@suse.de>

	PR debug/105158
	* tree-cfgcleanup.cc (move_debug_stmts_from_forwarder):
	Move debug stmts to the predecessor if moving to the
	destination is not possible.
	(remove_forwarder_block): Adjust.
	(remove_forwarder_block_with_phi): Likewise.