Set ISD::FROUND to Expand by default for all types

For most libm ISD nodes, TargetLoweringBase::initActions sets the default
scalar-type action to Expand, and leaves the vector-type action default as
Legal. This is not appropriate for the new ISD::FROUND node (which no backend
but PowerPC handles explicitly).

Fixes PR16842.

llvm-svn: 188048