Affine expr simplification for add of const multiple of same expression

- Detect "c_1 * expr + c_2 * expr" as (c_1 + c_2) * expr
- subsumes things like 'expr - expr' and "expr * -1 + expr" as 0.
- change AffineConstantExpr ctor to allow default null init

Signed-off-by: Uday Bondhugula <uday@polymagelabs.com>

Differential Revision: https://reviews.llvm.org/D76233