MultiValAssignOp occurs when an assignment operation (+=, *=, etc) does
not have single-valued left-hand or right-hand side.
Per the spec:
"In assignment operations, both the left- and right-hand expression lists
must contain exactly one single-valued expression"
Example:
func f() int {
x, y := 1, 2
x, y += 1
return x + y
}