[X86] select.ll - add ADD/SUB identity select test cases

LowerSELECTWithCmpZero already handles "SELECT (X != 0), Y, (OR/XOR Y, Z) -> (OR/XOR Y, (AND (0 - X), Z))" - we can also handle ADD/SUB in a similar way, although SUB is more restricted as its not commutative.

Author: RKSimon