[mlir] do not elide dialect prefix for ops with dots in the name

For the hypothetical "a.b.c" op printed within a region that declares "a" as
the default dialect, MLIR would currently elide the "a." prefix and only print
"b.c". However, this becomes ambiguous while parsing as "b.c" may be exist as
the "c" op in the "b" dialect. If it does not, the parsing currently fails. Do
not elide the default dialect if the op name contains further dots to avoid the
ambiguity.

See https://discourse.llvm.org/t/dropping-dialect-prefix-for-ops-with-multiple-dots-in-the-name/62562

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D125975