RFC: subreg fixing (extract_bit_field or simplify_subreg)

[David Edelsohn]

>>>>> Richard Henderson writes:

Richard> I thought about doing this when I was debugging the problem 
Richard> yesterday and was frightened by how often I expect it will
Richard> trigger.  *sigh*  I suppose it really is necessary.  Do it.

Richard> This is gonna be really ugly...

	Can we do this after GCC 4.0?  Is this really appropriate for stage3?

David