nir: Add fsin_agx opcode
Used to split up the fsin/fcos lowering for AGX between NIR and the backend, to permit algebraic optimizations without polluting NIR with too many hardware details. The backend NIR lowering produces an fmul/ffma of the input so we can optimize code like sin(2*x).
Passes relevant dEQP tests on G13G (the Apple M1). Discussed with @jekstrand on #dri-devel