Add more subreg offset helpers

[Eric Botcazou]

> subreg_offset_from_lsb was supposed to be the inverse operation of
> subreg_lsb, which also returns a bit number.

It would have helped the reviewer to state it in the function comment. ;-)

> Should I change that to return a byte number as well?

Both functions are fine as-is, but mention that the new one is the inverse of 
the old one in the comment.

-- 
Eric Botcazou