fix repeated tagged-to-i deopt

the comp_pivot variables were occasionally being used before being initialized.
the tagged-to-i operation would bail because of NaN being introduced when they
were used.

To test this, I modified the "simple.js" example to work as follows:

```javascript
var ndarray = require("ndarray")
var ndsort = require("../sort.js")
var unpack = require("ndarray-unpack")

var N = 100000;
var NX = 3;
var NY = 100;
var arrays = [];
//Create an array
for (var z = 0; z < N; ++z) { 
  var x = ndarray(new Float32Array(NY * NX), [NY, NX])

  for(var i=0; i<NY; ++i) {
    for(var j=0; j<NX; ++j) {
      x.set(i,j, Math.random())
    }
  }

  arrays.push(x);
}

//Sort x

for(var i = 0; i < N; ++i) {
  ndsort(arrays[i])
}
```

The remaining deopts appear to happen due to a combination of OSR and branching preventing full type collection. I roughly backported [this patch][] to v0.10 node to diagnose this. You can apply [my patch][] to a checkout of node v0.10 branch to get that info out of `--code-comments`.

Also, merry :christmas_tree:!

[this patch]: https://groups.google.com/forum/#!topic/v8-dev/laSb2A-hz5A
[my patch]: https://gist.github.com/chrisdickinson/d1232c5c3f15a27c95d2