[InstCombine] generalize subtract with 'not' operands; 2nd try

This is a re-try of 3aa009cc which was reverted at
9577fac0 because it caused an infinite loop.

For the extra test case, either re-ordering the transforms
or adding the extra clause to avoid sub-of-sub is enough
to prevent the infinite compile, but I'm doing both to be
safer.

Original commit message:
The motivation was to get min/max intrinsics to parity
with cmp+select idioms, but this unlocks a few more
folds because isFreeToInvert recognizes add/sub with
constants too.

In the min/max example, we have too many extra uses
for smaller folds to improve things, but this fold
is able to eliminate uses even though we can't reduce
the number of instructions.