Improve normalization performance for 32bit

- Uses a similar approach to the latest 64bit _normalize.
- Add one useful optimization back into the 64bit _normalize too.

Performance of 'bench' improved by around 0.5% for the 32bit field (but tested on a 64-bit machine).
The expression:

(t1 + 0x40 + ((t0 + 0x3D1) >> 26)) > 0x0x3FFFFFF
=
(t1 + ((t0 + 0x3D1) >> 26)) > 0x3FFFFBF
=
(t1<<26) + t0 + 3D1 > 0xFFFFEFC000000
=
t0 + (t1<<26) > 0xFFFFEFBFFFC2F

which is not exactly the lower bits of the characteristic. Sure this is right?