Move extensibility check to the top of Object.isFrozen/Object.isSealed

Move extensibility check to the top of Object.isFrozen/Object.isSealed

This speeds up isFrozen/isSealed checks on "normal" objects without
slowing down checks on frozen/sealed objects.

Though this ordering is not what ES5 specifies, the difference is not
observable (especially since the code bails out if the passed-in object
is a proxy).

Committed: https://code.google.com/p/v8/source/detail?r=13726

[rossberg, 2013-02-25 10:39:35]

lgtm

[adamk, 2013-02-25 19:44:25]

Committed patchset #2 manually as r13726 (presubmit successful).