Changes ConstantPoolArrayBuilder to do object lookups using the location
of the handles, rather than dereferencing the handles and comparing the
objects. This also updates CanonicalHandleScope when internalizing AST
nodes to ensure that duplicate objects share the same handles and so are
only added to the constant pool once.

BUG=v8:5203

Review-Url: https://codereview.chromium.org/2204243003
Cr-Commit-Position: refs/heads/master@{#38366}