Streamline spilling around calls in the code generator

[markshannon]

So that the GC can see all objects on the stack, any call that escapes and could thus potentially trigger the GC, must be wrapped in code to spill values to the stack and save the stack pointer.

Prior to tailcalling, it made sense to save and reload the stack pointer as the stack pointer would have usually been spilled anyway. Now that we use the preserve none calling convention, the stack pointer will not be spilled so reloading is redundant.

We can drop the reload and not adjust the stack pointer, merely saving the adjusted value.
For example, instead of:

    stack_pointer[0] = foo;
    stack_pointer[1] = bar;
    stack_pointer += 2;
    _PyFrame_SetStackPointer(frame, stack_pointer);
    escaping_call();
    stack_pointer = _PyFrame_GetStackPointer(frame);
    ...
    stack_pointer += -2;

we can do this:

    stack_pointer[0] = foo;
    stack_pointer[1] = bar;
    _PyFrame_SetStackPointer(frame, stack_pointer+2);
    escaping_call();
    ...

Which saves a few instructions on the hot path and reduces correction code on exit and error branches.