Refactor some code in SampleProfile.cpp

I'm adding new functionality in the sample profiler. This will
require more data to be kept around for each function, so I moved
the structure SampleProfile that we keep for each function into
a separate class.

There are no functional changes in this patch. It simply provides
a new home where to place all the new data that I need to propagate
weights through edges.

There are some other name and minor edits throughout.

llvm-svn: 195780