[llvm] [SelectionDAG] Disable FastISel for swiftasync functions (PR #70741)

[Felipe de Azevedo Piovezan via llvm-commits]

felipepiovezan wrote:

> Do you have any tests that you can add? Particularly the improved Argument lowering handling?

Yup, good thing you asked, because I've just found a bug with my `OptLevelChanger` version (the "if the optimization level changed" bit is problematic")

https://github.com/llvm/llvm-project/pull/70741