Refactored EXTERNAL_REFERENCE handling.

Previously, the result of target_reference_address() could only be
read, writing to it would have had an architecture-dependent effect,
e.g. writing into the code on ia32, a no-op on arm, etc.

This refactoring-only CL turns this into a simple getter, making it
impossible to use incorrectly.

More to come...

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/46583006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17467 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix double-boxing issue when materializing captured objects.

R=verwaest@chromium.org
TEST=mjsunit/compiler/escape-analysis-representation

Review URL: https://codereview.chromium.org/57783003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17468 ce2b1a6d-e550-0410-aec6-3dcde31c8c00