Move import population from threader to path solver.

Imports are our nomenclature for external SSA names to a block that
are used to calculate the outgoing edges for said block.  For example,
in the following snippet:

    <bb 2> :
    _1 = b_10 == block_11;
    _2 = b_10 != -1;
    _3 = _1 & _2;
    if (_3 != 0)
      goto <bb 3>; [INV]
    else
      goto <bb 5>; [INV]

...the imports to the block are b_10 and block_11 since they are both
needed to calculate _3.

The path solver takes a bitmap of imports in addition to the path
itself.  This sets up the number of SSA names to be on the lookout
for, while resolving the final conditional.

Calculating these imports was initially done in the threader, since it
was the only user of the path solver.  With new clients, it has become
obvious that populating the imports should be a task for the path
solver, so it can be shared among the clients.

This patch moves the import code to the solver, making both the solver
and the threader simpler in the process.  This is because intent is
clearer and some duplicate code was removed.

This reshuffling had the net effect of giving us a handful of new
threads through my suite of .ii files (125).  This was unexpected, but
welcome nevertheless.  There is no performance difference in callgrind
over the same suite.

Regstrapped on x86-64 Linux.

gcc/ChangeLog:

	* gimple-range-path.cc (path_range_query::add_copies_to_imports):
	Rename to...
	(path_range_query::compute_imports): ...this.  Adapt it so it can
	be passed the imports bitmap instead of working on m_imports.
	(path_range_query::compute_ranges): Call compute_imports in all
	cases unless an imports bitmap is passed.
	* gimple-range-path.h (path_range_query::compute_imports): New.
	(path_range_query::add_copies_to_imports): Remove.
	* tree-ssa-threadbackward.c (back_threader::resolve_def): Remove.
	(back_threader::find_paths_to_names): Inline resolve_def.
	(back_threader::find_paths): Call compute_imports.
	(back_threader::resolve_phi): Adjust comment.

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