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

[Adrian Prantl via llvm-commits]

https://github.com/adrian-prantl approved this pull request.

Given that this only affects -Onone, and only x86_64, I'm fine with this.

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