MIPS: Generators: Avoid calling into runtime if operand stack is empty

Port r15240 (909f716e)

Original commit message:
This patch makes yield sites save the resume continuation and context
inline.  If the operand stack is empty, we can avoid a call into the
runtime.  This also makes the SuspendJSGeneratorObject runtime function
less magical: it just has to save the operand stack and stack handlers.

This speeds up the following case by a factor of 3 or so:

  function* until(n) {
    for (var i = 0; i < n; i++)
      yield i;
  }

  function sum(iter) {
    var sum = 0;
    for (var x of iter) sum += x;
    return sum;
  }

  for (var i = 0; i < 10000; i++) sum(until(1000))

Also, there is no more sentinel value as the generators will resume in
the right place already, allowing me to remove the hack added to the
--debug-code check in r14437.

BUG=

Review URL: https://codereview.chromium.org/17291016

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15258 ce2b1a6d-e550-0410-aec6-3dcde31c8c00