swift234

[sil-simplify] Add a peephole for XOR

Use the following equality: (x xor y) xor y == x.
With this in mind (a xor b) xor c can be replaced by:
 a - if b and c are the same
 b - if a and c are the same
 c - if a and b are the same

rdar://20280322

Swift SVN r26568