Explicitly define ARMISelLowering::isFMAFasterThanFMulAndFAdd. No functionality change.

Currently ARM is the only backend that supports FMA instructions (for at least some subtargets) but does not implement this virtual, so FMAs are never generated except from explicit fma intrinsic calls. Apparently this is due to the fact that it supports both fused (one rounding step) and unfused (two rounding step) multiply + add instructions. This patch clarifies that this the case without changing behavior by implementing the virtual function to simply return false, as the default TargetLoweringBase version does.

It is possible that some cpus perform the fused version faster than the unfused version and vice-versa, so the function implementation should be revisited if hard data is found.