code: func (b *builder) assignOp(fn *Function, loc lvalue, incr Value, op token.Token, pos token.Pos) {
  	oldv := loc.load(fn)
  	loc.store(fn, emitArith(fn, op, oldv, emitConv(fn, incr, oldv.Type()), loc.typ(), pos))
  }
docs: // assignOp emits to fn code to perform loc += incr or loc -= incr.