[mlir][NFC] Switched `interfaces` to a private member of SSANameState.

`interfaces` is passed through to the `numberValuesIn*` functions with exactly
the same value as when SSANameState is constructed.  This just seems cleaner.

Also, a dependent PR adds `printerFlags` which follows similar code paths.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D105299