replace MachineCombinerPattern namespace and enum with enum class; NFCI

replace MachineCombinerPattern namespace and enum with enum class; NFCI

Also, remove an enum hack where enum values were used as indexes into an array.

We may want to make this a real class to allow pattern-based queries/customization (D13417).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252196 91177308-0d34-0410-b5e6-96231b3b80d8