multi-word operations

[David Edelsohn]

	Is this really just combine or is this also due to GCC not
tracking what part of a register could contain meaningful results after an
operation.  Yes, the compiler must widen but it also can know exactly what
effect widening will have and how that will interact with logical
operations. 

	Looking at GCC's PowerPC output for the same functions and the
intermediate optimization passes of IBM's XLC compiler, XLC seems to make
a lot of the gain by better knowing which operations are moot. 

David