Add a (currently) syntactic predicate to AST expression nodes telling
whether they are 'trivial'.  Trivial expressions have no side effects,
do not require storage to be allocated for them, and can be evaluated
out of order (because their value does not change between when they
are visited by the code generator as expressions in the AST and when
it is consumed).

Mark 'this' and literals as trivial.  Allow them to be pushed on the
virtual frame.  Make use of them to push 'this' more lazily in this
property assignments.

Review URL: http://codereview.chromium.org/647018

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3906 ce2b1a6d-e550-0410-aec6-3dcde31c8c00