arm load/store multiple

[Bernd Schmidt]

> If someone can think of a better way of representing this, so that some 
> pre-regalloc pass can combine loads in a way that will force the register 
> allocator to do the right thing, then I'm open to suggestions.

This is a problem I'm currently thinking about.  I'm not sure we want to do
this before register allocation.  It may work better to leave these
operations as single loads/stores until after reload, and then have a pass
that tries to combine multiple memory accesse into a stm/ldm.  This pass
would need to know how to reorder insns so the memory accesses are adjacent,
and it would need to know how to locally change the register allocations.
The advantage of doing it this way is that there is no risk of pessimizing
the code by forcing the register allocator to allocate groups of sequential
registers.

Bernd