[DAGCombiner] Improve division estimation of floating points.

[DAGCombiner] Improve division estimation of floating points.

Current implementation of estimating divisions loses precision since it
estimates reciprocal first and does multiplication.  This patch is to re-order
arithmetic operations in the last iteration in DAGCombiner to improve the
accuracy.

Reviewed By: Sanjay Patel, Jinsong Ji

Differential Revision: https://reviews.llvm.org/D66050

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@371713 91177308-0d34-0410-b5e6-96231b3b80d8