Change the discriminator assignment algorithm to make it more robust.

2013-05-24  Dehao Chen  <dehao@google.com>

	* gcc/testsuite/gcc.dg/debug/dwarf2/discriminator.c: New Testcase.
	* gcc/tree-cfg.c (locus_descrim_hasher::hash): Change discrminator
	hash function.
	(locus_descrim_hasher::equal): Likewise.
	(build_gimple_cfg): New discrminator assignmnet algorithm
	(make_edges): Likewise.
	(next_discriminator_for_locus): Likewise.
	(same_line_p): Likewise.
	(assign_discriminators): Likewise.
	(make_cond_expr_edges): Likewise.
	(make_gimple_switch_edges): Likewise.
	(make_goto_expr_edges): Likewise.
	(make_gimple_asm_edges): Likewise.

From-SVN: r199295