Previously, if a floating-point type was legal, but FNEG wasn't legal, we would use FSUB. Instead, we should use integer ops, to preserve the semantics. (Alternatively, there's a compiler-rt call we could use, but there isn't much reason to use that.)
It turns out we actually are still using this obscure SelectionDAG codepath in a few cases: on some targets, we have "legal" floating-point types that don't actually support any floating-point operations. In particular, ARM and AArch64 are using this path.
For GlobalISel, all targets without a native FNEG used this codepath.
The implementation for SelectionDAG is pretty simple because we can reuse the infrastructure from FCOPYSIGN. For GlobalISel, it's really easy because we can just blindly emit G_XOR and let legalization of the XOR take care of the rest.
See also 9a3dc3e, the corresponding change to type legalization.
Also includes a "bonus" change to STRICT_FSUB legalization, so we can lower an FSUB_STRICT to a float libcall.
Includes the changes to both LegalizeDAG and GlobalISel so we don't have inconsistent results in the future.