Calling OffsetOfElementAt becomes expensive when compiling functions with many
constant pool entries.  This was causing a regression in MandreelLatency due
to the time spent populating the constant pool array for large compiled
functions.

This change avoids calling OffsetOfElementAt for each entry, and instead keeps
track of the current offsets in ConstantPoolBuilder::Populate.  This gives the
following improvements on a Nexus 5:

                     Inline CP  |  OOL CP (before CL)  |  OOL CP (after CL)
Mandreel:               4305    |        3961          |       4120
MandreelLatency:        2298    |        1198          |       1994
Octane Score:           5197    |        4982          |       5152

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/376973002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22293 ce2b1a6d-e550-0410-aec6-3dcde31c8c00