Fix a small bug in conversion to SInt32

If the truncation from double returned out of bounds,
the code was not generating the right call to the fallback
method on 64 bit.

Change-Id: I6a126dd013c5b6373762beef0c1459fae1c26ef2
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Fix performance regression introduced by 6adb06

The global scope is compiled as EvalCode. Because of this
we were never using global lookups anymore, slowing down
the v8 test suite by ~20%.

Change-Id: I6c47ccf90f4d9ec3bf531bbb689d3f1511f69968
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>