Increase eval compilation cache hits.

Increase eval compilation cache hits.

This CL introduces two changes to the eval compilation cache. Firstly,
instead of checking for SharedFunctionInfo equivalence to ensure
equivalence of direct eval call sites we now check for Script and
scope start position equivalence.

Furthermore, we increase the cache hits by allowing to share global
eval code for direct and indirect call sites, e.g. in the case
  var x = eval; x("12"); eval("12");

BUG=v8:1854
TEST=existing tests in mjsunit

[Steven, 2011-12-08 10:00:19]

PTAL.