Create a wrapper pass for BlockFrequencyInfo.

Create a wrapper pass for BlockFrequencyInfo.

This is useful when we want to do block frequency analysis
conditionally (e.g. only in PGO mode) but don't want to add
one more pass dependence.

Patch by congh.
Approved by dexonsmith.
Differential Revision: http://reviews.llvm.org/D11196

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242248 91177308-0d34-0410-b5e6-96231b3b80d8