Reuse AstValueFactory when optimizing.

HOptimizedGraphBuilder::TryInline creates a temporary CompilationInfo and
destroys it, but we don't want the AstValueFactory to be destroyed at the same
time. Reuse the upper CompilationInfo's AstValueFactory.

BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/334173003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21851 ce2b1a6d-e550-0410-aec6-3dcde31c8c00