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[llvm][NFC] Add comments and common-case API to MachineBlockFrequencyInfo

Authored by mtrofin on Jul 23 2020, 8:42 AM.

Description

[llvm][NFC] Add comments and common-case API to MachineBlockFrequencyInfo

Clarify the relation between a block's BlockFrequency and the
getEntryFreq() API, and added an API for the relatively common case of
finding a block's frequency relative to the entrypoint.

Added / moved some comments to header.

Differential Revision: https://reviews.llvm.org/D84357