[PowerPC] Use the two-constant NR algorithm for refining estimates

The single-constant algorithm produces infinities on a lot of denormal values.
The precision of the two-constant algorithm is actually sufficient across the
range of denormals. We will switch to that algorithm for now to avoid the
infinities on denormals. In the future, we will re-evaluate the algorithm to
find the optimal one for PowerPC.

Differential revision: https://reviews.llvm.org/D60037

llvm-svn: 360144