The idea is from review of D87037, @spatel 's comments.
This is like FastMathFlagGuard in IR. Since we use SDAG instance to get values, it's with SelectionDAG. By creating a FlagInserter in current scope, all values created by getNode will get the flags, unless explicitly use Inserter->disable().
It's a prototype so in this revision I only enabled in visitFMA to show the same flag propagation as D87037.