fix($compile): remove comment nodes from templates before asserting single root node

The compiler will no longer throw if a directive template contains comment nodes in addition to a
single root node. If a template contains less than 2 nodes, the nodes are unaltered.

BREAKING CHANGE:

If a template contains directives within comment nodes, and there is more than a single node in the
template, those comment nodes are removed. The impact of this breaking change is expected to be
quite low.

Closes #9212
Closes #9215