Merged rules with ::placeholder pseudo-selectors broken in v3.3.7

[rolang]

This one looks similar to #561

.some-element {
    color: #d72319;
}
.some-other-element::placeholder {
    color: #d72319;
}

Gets merged to:

.some-element,.some-other-element::placeholder{color:#d72319}

which breaks the rule.

Should be:

.some-element{color: #d72319}.some-other-element::placeholder{color:#d72319}

[jakubpawlowicz]

We should treat ::placeholder selector as special too, see https:/jakubpawlowicz/clean-css/blob/master/lib/utils/compatibility.js#L23