Failure to reach fixpoint in 1st iteration when constant folding libcall

https://llvm.godbolt.org/z/456M4eq6f

We constant fold the call to `floor`, but it's not trivially dead so it hangs around after being RAUWd. Maybe we should mark the `TargetLibraryInfo::hasOptimizedCodeGen` functions as trivially dead?

Start by reproducing the failure with the Compiler Explorer example at https://llvm.godbolt.org/z/456M4eq6f and observe why the first constant-folding iteration fails to reach a fixpoint. Read the handling of floor and TargetLibraryInfo::hasOptimizedCodeGen, including how the RAUW'd call is classified for dead-code elimination. Done means the example reaches a fixpoint without hanging and the behavior is covered by an appropriate regression test.

Written by the indexing model from the issue text.