re PR rtl-optimization/2391 (Exponential compilation time explosion in combine)

        PR opt/2391
        * combine.c: Fix spelling in comment.
        (cached_nonzero_bits): New function.
        (cached_num_sign_bit_copies): New function.
	(nonzero_bits_with_known): New macro.
	(num_sign_bit_copies_with_known): New macro.
        (nonzero_bits1): Rename from nonzero_bits.  Add three new
	arguments.  Change calls from nonzero_bits to
	nonzero_bits_with_known.
        (num_sign_bit_copies1): Rename from num_sign_bit_copies.  Add
	three new arguments.  Change calls from num_sign_bit_copies to
	num_sign_bit_copies_with_known.
        (nonzero_bits): New macro.
        (num_sign_bit_copies): New macro.
        (update_table_tick): Don't traverse identical subexpression more
	than once.
        (get_last_value_validate): Likewise.

From-SVN: r62893