[instrprof][NFC] Refactor out the common logic for getProfileKind.

The logic for getProfileKind for RawInstrProfReader and
InstrProfReaderIndex is similar. To avoid duplication, move the logic
from the header to InstrProfReader.cpp and introduce a static method
which implements the common code.

Differential Revision: https://reviews.llvm.org/D118656