Make esi available to the register allocator rather than dedicating it
permanently to the context.

The context is still passed in register esi to JavaScript and to the runtime
as part of the calling convention.  Because some stubs might end up calling
JS or the runtime, it is also conservatively passed to stubs.

Roughly half the calls have been modified to use the context as an input
value in fixed register esi.  The other half are marked as calls or deferred
code so esi is spilled and can be explicitly set.

It is no longer necessary to restore the context to esi after a call that
might change it.

Review URL: http://codereview.chromium.org/6452001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6713 ce2b1a6d-e550-0410-aec6-3dcde31c8c00