[SelectionDAG] Add guard to automatically insert flags

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 if no Flags
argument provided.

In this patch, I applied it to floating point operations folding part in
DAG combiner, and removed Flags passing to getNode to show its effect.
Other places in DAG combiner and other helper methods similar to getNode
also need this. They can be done in follow-up patches.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D87361