[llvm-profdata] Add merge() to InstrProfRecord

[llvm-profdata] Add merge() to InstrProfRecord

Summary:
This change refactors two aspects of InstrProfRecord:

1) Add a merge() method to InstrProfRecord (previously InstrProfWriter combineInstrProfRecords()) in order to better encapsulate this functionality and to make the InstrProfRecord and SampleRecord APIs more consistent.

2) Make InstrProfRecord mergeValueProfData() a private method since it is only ever called internally by merge().

Reviewers: dnovillo, bogner, davidxl

Subscribers: silvas, vsk, llvm-commits

Differential Revision: http://reviews.llvm.org/D14786

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253695 91177308-0d34-0410-b5e6-96231b3b80d8