[mlir] Reintroduce API for creating operations with a DictionaryAttr

This patch reintroduces an API to create operations with a pre-existing
DictionaryAttr. This API does not populate the attributes with any
default attributes the operation may have, like the API that takes a
NamedAttrList does. NamedAttrList is effective at not re-hashing the
attributes if no default attributes were added, but this new API speeds
up clone-heavy workloads slightly (~5%).

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D144204