Add comments.

Author: jwanggit86

llvm/lib/Target/AMDGPU/VOP2Instructions.td

@@ -1966,6 +1966,8 @@ defm V_SUBREV_CO_CI_U32 :
 
 defm V_CVT_PK_RTZ_F16_F32  : VOP2_Real_FULL_with_name_gfx11_gfx12<0x02f,
   "V_CVT_PKRTZ_F16_F32", "v_cvt_pk_rtz_f16_f32">;
+// FIXME: V_PK_FMAC_F16 is currently not used in instruction selection.
+// If this changes, ensure the DPP variant is not used for GFX11+.
 defm V_PK_FMAC_F16     : VOP2_Real_e32_gfx11_gfx12<0x03c>;
 
 defm V_ADD_F16             : VOP2_Real_FULL_t16_and_fake16_gfx11_gfx12<0x032, "v_add_f16">;