Revert 250343 and 250344

Turns out this approach is buggy.  In discussion about follow on work, Sanjoy pointed out that we could be subject to circular logic problems.  

Consider:
 if (i u< L) leave()
 if ((i + 1) u< L) leave()
 print(a[i] + a[i+1]) 

If we know that L is less than UINT_MAX, we could possible prove (in a control dependent way) that i + 1 does not overflow.  This gives us:
 if (i u< L) leave()
 if ((i +nuw 1) u< L) leave()
 print(a[i] + a[i+1]) 

If we now do the transform this patch proposed, we end up with:
 if ((i +nuw 1) u< L) leave_appropriately()
 print(a[i] + a[i+1]) 

That would be a miscompile when i==-1.  The problem here is that the control dependent nuw bits got used to prove something about the first condition.  That's obviously invalid.

This won't happen today, but since I plan to enhance LVI/CVP with exactly that transform at some point in the not too distant future...

llvm-svn: 250430