InstrProf: Avoid linear search in a hot loop

Every time we were adding or removing an expression when generating a
coverage mapping we were doing a linear search to try and deduplicate
the list. The indices in the list are important, so we can't just
replace it by a DenseMap entirely, but an auxilliary DenseMap for fast
lookup massively improves the performance issues I was seeing here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218892 91177308-0d34-0410-b5e6-96231b3b80d8