can't run combine after flow.

[Geoffrey Keating <geoffk at cygnus dot com>]

gcc doesn't currently bootstrap because of this change:

	* toplev.c (rest_of_compilation): Rebuild jump labels if
	combine_instructions has created a new direct jump.

The problem is this:

after flow1, the code looks like this:

(jump_insn 65 64 173 (set (pc)
        (if_then_else (eq (reg:CC 97)
                (const_int 0 [0x0]))
            (label_ref 81)
            (pc))) 853 {jump-4} (insn_list 64 (nil))
    (expr_list:REG_DEAD (reg:CC 97)
        (nil)))
;; End of basic block 0

;; Start of basic block 1, registers live: 1 [1] 2 [2] 31 [31] 82 83 85
(note 173 65 67 [bb 1] NOTE_INSN_BASIC_BLOCK)

(insn 67 173 73 (set (reg:SI 98)
        (mem/s:SI (plus:SI (reg/v:SI 82)
                (const_int 36 [0x24])) 15)) 532 {movsi+1} (nil)
    (nil))
...

combine determines that the condition is never false
(by noticing that the test is of a bitfield extract of a bitfield
that has previously been cleared).  So it changes the jump
insn to be unconditional.

Then when jump labels are rebuilt, jump calls

  delete_barrier_successors (f);

which naturally deletes all of basic block 1.

This is probably a sign that we need to fix delete_barrier_successors
etc. in jump to play nice with the flow information; we certainly want
to do the deletion at this point rather than after reload!

-- 
Geoff Keating <geoffk@cygnus.com>