[DAG Combiner] Fix the native computation of the Newton series for reciprocals

[DAG Combiner] Fix the native computation of the Newton series for reciprocals

The generic infrastructure to compute the Newton series for reciprocal and
reciprocal square root was conceived to allow a target to compute the series
itself.  However, the original code did not properly consider this condition
if returned by a target.  This patch addresses the issues to allow a target
to compute the series on its own.

Differential revision: https://reviews.llvm.org/D22975

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286523 91177308-0d34-0410-b5e6-96231b3b80d8