[mlir] Refactoring a few Parser APIs

Refactored two new parser APIs parseGenericOperationAfterOperands and
 parseCustomOperationName out of parseGenericOperation and parseCustomOperation.

Motivation: Sometimes an op can be printed in a special way if certain criteria
is met. While parsing, we need to handle all the versions.
`parseGenericOperationAfterOperands` is handy in situation where we already
parsed the operands and decide to fall back to default parsing.

`parseCustomOperationName` is useful when we need to know details (dialect,
operation name etc.) about a parsed token meant to be an mlir operation.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D113719