[llvm] [FastISel] Don't force SDAG fallback for libcalls (PR #171782)

[Nikita Popov via llvm-commits]

nikic wrote:

Huh, looks like these fallbacks did actually have significant cost: https://llvm-compile-time-tracker.com/compare.php?from=296781524902c4eb06efc5bb054cac58b973d839&to=d33d80fae606104bb82673d8a17657fe99afef70&stat=instructions:u

https://github.com/llvm/llvm-project/pull/171782