loop patch #7

[Richard Henderson]

Highlights:

 - init_alias_analysis change reverted; must have been user error, since
   I can no longer reproduce the problem that prompted me to change it.

 - Combine bivs with the same increment by converting all but one to a giv.
   Don't do this for bivs used in loop conditions because unroll can't cope.

The lossage I'd reported on loop startup for resid I now attribute to
the fact that TYPE_SIZE is in bits, and cse can't unify N with N*B/B.


r~