[mlir] Optimize OperationName construction and usage

When constructing an OperationName, the overwhelming majority of
cases are from registered operations. This revision adds a non-locked
lookup into the currently registered operations, which prevents locking
in the common case. This revision also optimizes several uses of
RegisteredOperationName that expect the operation to be registered,
e.g. such as in OpBuilder.

These changes provides a reasonable speedup (5-10%) in some
compilations, especially on platforms where locking is expensive.

Differential Revision: https://reviews.llvm.org/D117187