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

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=

Committed: https://code.google.com/p/v8/source/detail?r=15258

[Paul Lind, 2013-06-20 23:41:55]

lgtm

[palfia, 2013-06-20 23:43:18]

Committed manually as r15258 (presubmit successful).