[DAGCombiner] Set disjoint flag in add->or and xor->or combines (#86925)

We check DAG.haveNoCommonBitsSet so the operands will be known to be
disjoint.

I couldn't think of a codegen test case since most targets aren't
checking hasDisjoint yet, apart from RISCV in the or_is_add pattern, but
it also falls back to computeKnownBits.