[DAGCombiner] Rebuild (setcc x, y, ==) from (xor (xor x, y), 1)

[DAGCombiner] Rebuild (setcc x, y, ==) from (xor (xor x, y), 1)

The existing code already considered this case. Unfortunately a typo in
the condition prevents it from triggering. Also the existing code, had
it run, forgot to do the folding.

This fixes PR42876.

Differential Revision: https://reviews.llvm.org/D65802