DWARF: turn dw_loc_descr_node field into hash map for frame offset check

DWARF: turn dw_loc_descr_node field into hash map for frame offset check

As discussed on
<https://gcc.gnu.org/ml/gcc-patches/2016-02/msg01708.html>, this change
removes a field in the dw_loc_descr_node structure so we can get rid of
the CHECKING_P macro usage.

This field was used to perform consistency checks for frame offset in
DWARF procedures. As a replacement, this commit turns the "visited
nodes" set in resolve_args_picking_1 into a map that remembers for each
dw_loc_descr_node the frame offset associated to it, so that the
consistency check is still operational.

Boostrapped and regtested on x86_64-linux.

2016-04-27  Pierre-Marie de Rodat  <derodat@adacore.com>

	* dwarf2out.h (struct dw_loc_descr_node): Remove the
	dw_loc_frame_offset field.
	* dwarf2out.c (new_loc_descr): Likewise.
	(resolve_args_picking_1): Turn the VISITED hash set into a
	FRAME_OFFSET hash map. Use it to associate a frame offset to
	visited nodes. Remove uses of the CHECKING_P macro.
	(resolve_args_picking): Update call to resolve_args_picking_1.

From-SVN: r235515