Patch. Fix redundent copies between nodes with different but compatible register classes

[Tim Northover]

Hi Yin,

> This patch will remove the copy between def-use nodes with two different but
> compatible register classes.

What's the purpose of this patch? If it's purely to decrease
compile-time, then it may be ok. If it's an actual change to codegen:

1. Shouldn't register-coalescing be where this is dealt with? That's
the code which is designed to see through super/sub-register copies.
2. It presumably has an effect on code. Could we have a test?

Cheers.

Tim.