PGO: Scale large counters down to 32-bits

PGO counters are 64-bit and branch weights are 32-bit.  Scale them down
when necessary, instead of just taking the lower 32 bits.

<rdar://problem/16276448>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203592 91177308-0d34-0410-b5e6-96231b3b80d8
PGO: Clarify the testing README

<rdar://problem/16276448>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203593 91177308-0d34-0410-b5e6-96231b3b80d8