[GlobalISel][CallLowering] NFC: Unify flag-setting from CallBase + AttributeList
It's annoying to have to maintain multiple, nearly identical chains of if
statements which all set the same attributes.
Add a helper function, addFlagsUsingAttrFn which performs the attribute
Then, use wrappers for that function in lowerCall and setArgFlags.
(Note that the flag-setting code in setArgFlags was missing the returned
attribute. There's no selection for this yet, so no test. It's an example of
the kind of thing this lets us avoid, though.)
Differential Revision: https://reviews.llvm.org/D86159