[NFC][BlockFrequency] Move operator definitions into header

While BlockFrequency::operator+= is a very simple operation, it's
definition is present in another TU which means that it doesn't get
inlined in non-LTO builds. This means that there is some performance
left on the table in those builds, as this operator is called many
times.

This patch moves that operator (and a few others) into the
BlockFrequency.h header which gives a small speedup (~0.1%):
https://llvm-compile-time-tracker.com/compare.php?from=6ee594be53e7efaa12086ad20f0d0268092a4c73&to=6ac6cd99e211fae5ae5de41ad608604aa22f1882&stat=instructions%3Au

Differential Revision: https://reviews.llvm.org/D152781