Allow loading constant function from proto chain.

This enables Crankshaft to use HConstant for loading constant functions
on the prototype chain when building a monomorphic load. This pattern
appears in several JavaScript frameworks.

R=svenpanne@chromium.org
TEST=mjsunit/compiler/proto-chain-constant

Review URL: https://codereview.chromium.org/12052008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13463 ce2b1a6d-e550-0410-aec6-3dcde31c8c00