tree-optimization/103596 - fix missed propagation into switches

tree-optimization/103596 - fix missed propagation into switches

may_propagate_copy unnecessarily restricts propagating non-abnormals
into places that currently contain an abnormal SSA name but are
not the PHI argument for an abnormal edge.  This causes VN to
not elide a CFG path that it assumes is elided, resulting in
released SSA names in the IL.

The fix is to enhance the may_propagate_copy API to specify the
destination is _not_ a PHI argument.  I chose to not update only
the relevant caller in VN and the may_propagate_copy_into_stmt API
at this point because this is a regression and needs backporting.

2021-12-07  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/103596
	* tree-ssa-sccvn.c (eliminate_dom_walker::eliminate_stmt):
	Note we are not propagating into a PHI argument to may_propagate_copy.
	* tree-ssa-propagate.h (may_propagate_copy): Add
	argument specifying whether we propagate into a PHI arg.
	* tree-ssa-propagate.c (may_propagate_copy): Likewise.
	When not doing so we can replace an abnormal with
	something else.
	(may_propagate_into_stmt): Update may_propagate_copy calls.
	(replace_exp_1): Move propagation checking code to
	propagate_value and rename to ...
	(replace_exp): ... this and elide previous wrapper.
	(propagate_value): Perform checking with adjusted
	may_propagate_copy call and dispatch to replace_exp.

	* gcc.dg/torture/pr103596.c: New testcase.