[llvm] [LLVM] Slay undead copysign code (PR #111269)

[via llvm-commits]

workingjubilee wrote:

@topperc This is using `report_fatal_error` now.

Also, good news: I seem to have found the spot where SelectionDAG expands copysign that catches this before we ever hit the libcall legalization: https://github.com/llvm/llvm-project/blob/75103aae4a9d22e2c46068f2160f2dddd6ad2116/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp#L1646-L1707

https://github.com/llvm/llvm-project/pull/111269