tree-optimization/106514 - revisit m_import compute in backward threading

This revisits how we compute imports later used for the ranger path
query during backwards threading.  The compute_imports function
of the path solver ends up pulling the SSA def chain of regular
stmts without limit and since it starts with just the gori imports
of the path exit it misses some interesting names to translate
during path discovery.  In fact with a still empty path this
compute_imports function looks like not the correct tool.

The following instead implements what it does during the path discovery
and since we add the exit block we seed the initial imports and
interesting names from just the exit conditional.  When we then
process interesting names (aka imports we did not yet see the definition
of) we prune local defs but add their uses in a similar way as
compute_imports would have done.

compute_imports also is lacking in its walking of the def chain
compared to range_def_chain::get_def_chain which for example
handles &_1->x specially through range_op_handler and
gimple_range_operand1, so the code copies this.  A fix for
compute_imports will be done separately, also fixing the unbound
walk there.

The patch also properly unwinds m_imports during the path discovery
backtracking and from a debugging session I have verified the two
sets evolve as expected now while previously behaving slightly erratic.

Fortunately the m_imports set now also is shrunken significantly for
the PR69592 testcase (aka PR106514) so that there's overall speedup
when increasing --param max-jump-thread-duplication-stmts as
15 -> 30 -> 60 -> 120 from 1s -> 2s -> 13s -> 27s to with the patch
1s -> 2s -> 4s -> 8s.

This runs into a latent issue in X which doesn't seem to expect
any PHI nodes with a constant argument on an edge inside the path.
But we now have those as interesting, for example for the ICEing
g++.dg/torture/pr100925.C which just has sth like

  if (i)
    x = 1;
  else
    x = 5;
  if (x == 1)
    ...

where we now have the path from if (i) to if (x) and the PHI for x
in the set of imports to consider for resolving x == 1 which IMHO
looks exactly like what we want.  The path_range_query::ssa_range_in_phi
papers over the issue and drops the range to varying instead of
crashing.  I didn't want to mess with this any further in this patch
(but I couldn't resist replacing the loop over PHI args with
PHI_ARG_DEF_FROM_EDGE, so mind the re-indenting).

	PR tree-optimization/106514
	* tree-ssa-threadbackward.cc (back_threader::find_paths_to_names):
	Compute and unwind both m_imports and interesting on the fly during
	path discovery.
	(back_threader::find_paths): Compute the original m_imports
	from just the SSA uses of the exit conditional.  Drop
	handling single_succ_to_potentially_threadable_block.
	* gimple-range-path.cc (path_range_query::ssa_range_in_phi): Handle
	constant PHI arguments without crashing.  Use PHI_ARG_DEF_FROM_EDGE.

	* gcc.dg/tree-ssa/ssa-thread-19.c: Un-XFAIL.
	* gcc.dg/tree-ssa/ssa-thread-20.c: New testcase.

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