[InstSimplify] fold xor logic of 2 variables, part 2

(~a & b) ^ (a | b) --> a

This is the swapped and/or (Demorgan?) sibling fold for
the fold added with D114462 ( 892648b1 ).

This case is easier to specify because we are returning
a root value, not a 'not':
https://alive2.llvm.org/ce/z/SRzj4f