Make the PPC fast-math sqrt expansion safe at 0

In fast-math mode sqrt(x) is calculated using the fast expansion of the
reciprocal of the reciprocal sqrt expansion. The reciprocal and reciprocal
sqrt expansions use the associated estimate instructions along with some Newton
iterations. Unfortunately, as a result, sqrt(0) was being calculated as NaN,
which is not correct. Now we explicitly return a result of zero if the input is
zero.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190624 91177308-0d34-0410-b5e6-96231b3b80d8