Some AST nodes (Property, Call, etc.) store either a list of receiver
types or a monomorphic receiver type. This patch merges the two fields
using a small pointer list. GetMonomorphicReceiverType() is now a
purely convenience function returning the first and only recorded
type.

This saves about 500K (of about 39M) on average when compiling V8
benchmark as measured by a simple patch adding a zone allocation
counter (https://gist.github.com/1149397).

R=kmillikin@chromium.org

Review URL: http://codereview.chromium.org/7655017

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8993 ce2b1a6d-e550-0410-aec6-3dcde31c8c00