[SelectionDAG] Fix bugs in inverted condition splitting code.

Summary:
Fix two bugs in SelectionDAGBuilder::FindMergedConditions reported by
Mikael Holmen.  Handle non-canonicalized xor not operation
correctly (was assuming operand 0 was always the non-constant operand)
and check that the negated condition is also in the same block as the
original and/or instruction (as is done for and/or operands already)
before proceeding with optimization.

Reviewers: bogner, MatzeB, qcolombet

Subscribers: mcrosier, uabelho, llvm-commits

Differential Revision: https://reviews.llvm.org/D29680

llvm-svn: 294605