Fix quadratic behavior in sequence numbering.

The ir::layout module is assigning sequence numbers to all EBBs and
instructions so relative positions can be computed in constant time.
This works a lot like BASIC line numbers where we initially use numbers
10, 20, 30, ... so we can insert new instructions in the middle of the
sequence without renumbering everything.

In some cases where the coalescer is misbehaving and inserting a lot of
copy instructions, we end up having to renumber a larger and larger
number of instructions to make space in the sequence. This causes the
following reload pass to be very slow, spending most of its time
renumbering instructions.

Fix this by putting an upper limit on the number of instructions we're
willing to renumber locally. When the limit is reached, switch to a full
function renumbering with the major stride of 10. This gives us new
elasticity in the sequence numbers.

- Time to compile the Python interpreter in #229 drops from 4826 s -> 15.8 s.
- The godot benchmark in #226 drops from 1257 s -> 75 s.
- The AngryBots1 benchmark does not have the coalescer misbehavior.
  Its compilation time changes 22.9 s -> 23.1 s.

It's worth noting that the sequence numbering is still technically
quadratic with this fix. The system is not designed to handle a large
number of instructions inserted in a single location. It expects a more
even distribution of new instructions.

We still need to fix the coalescer. It should not insert so many copies
in degenerate cases.

lib/cretonne/src/timing.rs

@@ -69,6 +69,7 @@ define_passes!{
 
     prologue_epilogue: "Prologue/epilogue insertion",
     binemit: "Binary machine code emission",
+    layout_renumber: "Layout full renumbering",
 }
 
 impl Pass {