Change dialect `printOperation()` hook to `getOperationPrinter()`

This makes the hook return a printer if available, instead of using LogicalResult  to
indicate if a printer was available (and invoked). This allows the caller to detect that
the dialect has a printer for a given operation without actually invoking the printer.
It'll be leveraged in a future revision to move printing the op name itself under control
of the ASMPrinter.

Differential Revision: https://reviews.llvm.org/D108803