[AArch64] Fix the number of iterations for the Newton series

The number of iterations was incorrectly determined for DP FP vector types
and the tests were insufficient to flag this issue.

Differential revision: https://reviews.llvm.org/D39507

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317349 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed