[SelectionDAG] Add helper guard to automatically insert flags

[qiucf]

Summary

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.

[steven.zhang]

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

4413

This is not right as you override the Flags secretly. How about having another version that didn't have the Flags parameter and we will get it from Inserter. And one version that has the Flags if people really want to set the flags manually.(I wonder if there is such case)

[qiucf]

Keep flags in current methods and introduce new methods to auto-insert flags. (maybe can use variadic template methods?)

[qiucf]

Remove flag passing in methods visiting floating-point nodes.

[spatel]

LGTM - this makes it more likely that we propagate as expected and cleans up the current code.
We probably have more cases like D88063, but that can be handled as follow-up patches.