[ADT] Use try_emplace in SCCIterator.h (NFC) (#164187)
This patch replaces an unidiomatic operator[] usage with try_emplace
to ensure in-place NodeInfo construction and correct Group field
initialization.
This patch also clarifies the comment. In particular, "insertion
operation" by itself isn't clear whether it's referring to insert() or
all of insertion operations like insert(), insert_or_assign(),
try_emplace(), etc.
