[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
setting.

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