Whether an instruction is deemed to have side effects in determined by whether it has an instruction pattern that emits a single instruction. Because of the way a lot of the the vcvt instructions are specified either in dagtodag code or with tblgen patterns that emit multiple instructions, they don't get marked as not having side effects.
This just marks them as not having side effects manually. It can help especially with instruction scheduling, to not create artificial barriers, but one of these tests also managed to produce fewer instructions.