[mlir][NFC] Make 'printOp' public in AsmPrinter

This patch moves the 'printOp' functionality to the public API of
AsmPrinter and rename it to 'printCustomOrGenericOp'. No 'parseOp'
is needed at this time as existing APIs are able to parse operations
producing results where results are omitted in the textual form
(the LHS of an operation is redundant when it comes to building the
operation itself as it only contains the result names).

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D135006