Fix FMA instruction on LLVM

We were emitting the wrong instructions for fused multiply-add operations on LLVM:

• the instruction name is llvm.fma.f32 or llvm.fma.f64, not fmadd
• LLVM does not support other instructions such as fmsub; instead we implement these by flipping signs of some arguments
• the instruction is an LLVM intrinsic, which requires handling it like a normal function call instead of a machine instruction