tree-ssa-dce: Fix up -fcompare-debug failures in make_forwarders_with_degenerate_phis [PR103742]

make_forwarders_with_degenerate_phis causes a -fcompare-debug failure on the
following testcase.
The problem is that on:
  # iftmp.4_8 = PHI <&D.2582(6), &D.2583(4), &D.2582(7), &D.2583(5)>
the exact DECL_UIDs are different between -g and -g0 (which is ok, with -g
the decls can have larger gaps in between the uids), which means
iterative_hash_expr is different and because there are 2 pairs of edges
with matching phi arguments, the function processes them in different
orders.
The following patch fixes it by using the iterative_hash_expr order
only to determine which arguments are the same, then replaces the hashes
with the minimum dest_idx in the set of matching arguments and qsorts
again (which makes it stable for -fcompare-debug) and only splits edges etc.
on that stable order.
As a small optimization, if no arguments are equal, it doesn't do the
second qsort and continues, and if all arguments of the PHI are
constants or SSA_NAMEs (I think that is a pretty common case for many
PHIs), then it doesn't do the second qsort either, because in that case
the hash values will be stable, only computed from the constant values or
SSA_NAME_VERSIONs.

2021-12-29  Jakub Jelinek  <jakub@redhat.com>

	PR debug/103742
	* tree-ssa-dce.c (make_forwarders_with_degenerate_phis): If any phi
	argument is not CONSTANT_CLASS_P or SSA_NAME and any arguments are
	equal, change second from hash value to lowest dest_idx from the
	edges which have equal argument and resort to ensure -fcompare-debug
	stability.

	* g++.dg/opt/pr103742.C: New test.