[APFloat] Added isFiniteNonZero predicate.

This is the first patch in a series of patches to rename isNormal =>
isFiniteNonZero and isIEEENormal => isNormal. In order to prevent careless
errors on my part the overall plan is:

1. Add the isFiniteNonZero predicate with tests. I can do this in a method
independent of isNormal. (This step is this patch).
2. Convert all references to isNormal with isFiniteNonZero. My plan is to
comment out isNormal locally and continually convert isNormal references =>
isFiniteNonZero until llvm/clang compiles.
3. Remove old isNormal and rename isIEEENormal to isNormal.
4. Look through all of said references from patch 2 and see if we can simplify
them by using the new isNormal.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184350 91177308-0d34-0410-b5e6-96231b3b80d8