[turbofan] Try hard(er) to use smi representation for float64 values.

[turbofan] Try hard(er) to use smi representation for float64 values.

Previously ChangeLowering would always box float64 values when going to
tagged representation, but that introduces a lot of deoptimizer loops
and polymorphism into TurboFan, which is unfortunate and unnecessary.

This adds some logic to ChangeFloat64ToTagged to try harder to create a
Smi when going from Float64 to Tagged, instead of always allocating a
HeapNumber.  This might need some additional tweaking, but at least it
makes it possible to start comparing TurboFan and Crankshaft for some
regular JavaScript.

R=jarin@chromium.org

Committed: https://crrev.com/7709e41aec9de8b42ddebb197a02c1017d6fe87a
Cr-Commit-Position: refs/heads/master@{#31609}

[Benedikt Meurer, 2015-10-27 14:03:43]

Jaro: PTAL
Danno: FYI

[Jarin, 2015-10-27 15:17:03]

lgtm

https://codereview.chromium.org/1420913003/diff/20001/src/compiler/change-low...
File src/compiler/change-lowering.cc (right):

https://codereview.chromium.org/1420913003/diff/20001/src/compiler/change-low...
src/compiler/change-lowering.cc:225: return Replace(value);
Just wondering - is there a reason to not use select here?

[Benedikt Meurer, 2015-10-27 16:16:19]

https://codereview.chromium.org/1420913003/diff/20001/src/compiler/change-low...
File src/compiler/change-lowering.cc (right):

https://codereview.chromium.org/1420913003/diff/20001/src/compiler/change-low...
src/compiler/change-lowering.cc:225: return Replace(value);
Allocate requires a control input.

[Benedikt Meurer, 2015-10-27 16:16:23]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-10-27 16:16:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1420913003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1420913003/20001

[commit-bot: I haz the power, 2015-10-27 16:17:48]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-27 16:18:29]

Patchset 2 (id:??) landed as
https://crrev.com/7709e41aec9de8b42ddebb197a02c1017d6fe87a
Cr-Commit-Position: refs/heads/master@{#31609}