Split SelectionDAGISel::IsLegalAndProfitableToFold to

IsLegalToFold and IsProfitableToFold. The generic version of the later simply checks whether the folding candidate has a single use.

This allows the target isel routines more flexibility in deciding whether folding makes sense. The specific case we are interested in is folding constant pool loads with multiple uses.

llvm-svn: 96255