[ScalarEvolution] Fix pointer/int type handling converting select/phi to min/max.

The old version of this code would blindly perform arithmetic without
paying attention to whether the types involved were pointers or
integers.  This could lead to weird expressions like negating a pointer.

Explicitly handle simple cases involving pointers, like "x < y ? x : y".
In all other cases, coerce the operands of the comparison to integer
types.  This avoids the weird cases, while handling most of the
interesting cases.

Differential Revision: https://reviews.llvm.org/D103660