e.(StrConst).isDocString() can only hold if e instanceof StrConst, since we have
that condition on the line above, we can safely remove this condition.
This used to take 30s on `cpython`.
```
Tuple counts for StatementNoEffect::side_effecting_binary#f:
46522 ~0% {2} r1 = ClassObject::ClassObject::hasAttribute_dispred#fb AS L AND NOT StatementNoEffect::side_effecting_binary#f#antijoin_rhs AS R(L.<0>, L.<1>)
46522 ~2% {2} r2 = SCAN r1 OUTPUT r1.<1>, r1.<0>
950960 ~2% {2} r3 = JOIN r2 WITH Operations::Operator::getSpecialMethodName_dispred#ff_10#join_rhs AS R ON FIRST 1 OUTPUT R.<1>, r2.<1>
950960 ~2% {2} r4 = JOIN r3 WITH py_operators AS R ON FIRST 1 OUTPUT R.<2>, r3.<1>
950960 ~0% {3} r5 = JOIN r4 WITH AstGenerated::BinaryExpr_::getLeft_dispred#ff AS R ON FIRST 1 OUTPUT R.<1>, r4.<1>, r4.<0>
122934382 ~0% {2} r6 = JOIN r2 WITH Operations::Cmpop::getSpecialMethodName_dispred#ff_10#join_rhs AS R ON FIRST 1 OUTPUT R.<1>, r2.<1>
122934382 ~3% {3} r7 = JOIN r6 WITH project#Operations::Compare::compares_dispred#ffff#3_201#join_rhs AS R ON FIRST 1 OUTPUT R.<2>, r6.<1>, R.<1>
123885342 ~3% {3} r8 = r5 \/ r7
300 ~8% {1} r9 = JOIN r8 WITH project#Exprs::Expr::refersTo_dispred#ffff AS R ON FIRST 2 OUTPUT r8.<2>
return r9
```
With this commit, it takes a few milliseconds.
