[PowerPC][NFC] refactor eligible check for tail call optimization

The check logic for TCO is scattered in two functions:
IsEligibleForTailCallOptimization_64SVR4() IsEligibleForTailCallOptimization(),
and serves instruction selection phase only at this moment.

This patch aims to refactor existing logic to export an API for TCO
eligible query before instruction selection phase.

Reviewed By: shchenz, nemanjai

Differential Revision: https://reviews.llvm.org/D141673