[llvm][NFC] refactor setBlockFrequency for clarity.

The refactoring encapsulates frequency calculation in MachineBlockFrequencyInfo,
and renames the API to clarify its motivation. It should clarify
frequencies may not be reset 'freely' by users of the analysis, as the
API serves as a partial update to avoid a full analysis recomputation.

Differential Revision: https://reviews.llvm.org/D84427