[APFloat] Rename isIEEENormal => isNormal and remove old isNormal method.

The old isNormal is already functionally replaced by the method isFiniteNonZero
in r184350 and all references to said method were replaced in LLVM/clang in
r184356/134366.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184449 91177308-0d34-0410-b5e6-96231b3b80d8