[APFloat] Move setting fcNormal in zeroSignificand() to calling code.

[APFloat] Move setting fcNormal in zeroSignificand() to calling code.

Zeroing the significand of a floating point number does not necessarily cause a
floating point number to become finite non zero. For instance, if one has a NaN,
zeroing the significand will cause it to become +/- infinity.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187313 91177308-0d34-0410-b5e6-96231b3b80d8