Convert cost table lookup functions to return a pointer to the entry or nullptr inste...

Convert cost table lookup functions to return a pointer to the entry or nullptr instead of the index.

This avoid mentioning the table name an extra time and allows the lookup to be done directly in the ifs by relying on the bool conversion of the pointer.

While there make use of ArrayRef and std::find_if.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251382 91177308-0d34-0410-b5e6-96231b3b80d8