[turbofan] Combine JSToNumber and ChangeTaggedToFloat64 for fast SMI case.

[turbofan] Combine JSToNumber and ChangeTaggedToFloat64 for fast SMI case.

This does the following optimization during ChangeLowering:

 ChangeTaggedToFloat64(JSToNumber(x)) =>
   if IsSmi(x) then ChangeSmiToFloat64(x)
   else let y = JSToNumber(x) in
     if IsSmi(y) then ChangeSmiToFloat64(y)
     else LoadHeapNumberValue(y)

Also cleanup/rearrange some code duplication in ChangeLowering.

R=dcarney@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=25195

[Benedikt Meurer, 2014-11-06 13:19:17]

bmeurer@chromium.org changed reviewers:
+ dcarney@chromium.org

[Benedikt Meurer, 2014-11-06 13:19:18]

PTAL

[dcarney, 2014-11-06 13:24:25]

lgtm

[Benedikt Meurer, 2014-11-06 13:25:51]

Committed patchset #1 (id:1) manually as 25195 (presubmit successful).