tree-into-ssa: speed up sorting in prune_unused_phi_nodes [PR114480]

tree-into-ssa: speed up sorting in prune_unused_phi_nodes [PR114480]

In PR 114480 we are hitting a case where tree-into-ssa scales
quadratically due to prune_unused_phi_nodes doing O(N log N)
work for N basic blocks, for each variable individually.
Sorting the 'defs' array is especially costly.

It is possible to assist gcc_qsort by laying out dfs_out entries
in the reverse order in the 'defs' array, starting from its tail.
This is not always a win (in fact it flips most of 7-element qsorts
in this testcase from 9 comparisons (best case) to 15 (worst case)),
but overall it helps on the testcase and on libstdc++ build.
On the testcase we go from 1.28e9 comparator invocations to 1.05e9,
on libstdc++ from 2.91e6 to 2.84e6.

gcc/ChangeLog:

	PR c++/114480
	* tree-into-ssa.cc (prune_unused_phi_nodes): Add dfs_out entries
	to the 'defs' array in the reverse order.