PPC: Improve code generation for mixed-precision reciprocal sqrt

The DAGCombine logic that recognized a/sqrt(b) and transformed it into
a multiplication by the reciprocal sqrt did not handle cases where the
sqrt and the division were separated by an fpext or fptrunc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178801 91177308-0d34-0410-b5e6-96231b3b80d8