[DAGCombine] Fold redundant select

Recommit bbdf2435.

Original commit message:

If a chain of two selects share a true/false value and are controlled
by two setcc nodes, that are never both true, we can fold away one of
the selects. So, the following:
(select (setcc X, const0, eq), Y,
  (select (setcc X, const1, eq), Z, Y))

Can be combined to:
  select (setcc X, const1, eq) Z, Y

Differential Revision: https://reviews.llvm.org/D142535