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).

llvm-svn: 252196
diff --git a/llvm/lib/CodeGen/MachineCombiner.cpp b/llvm/lib/CodeGen/MachineCombiner.cpp
index bfa5a6a..11b6a79 100644
--- a/llvm/lib/CodeGen/MachineCombiner.cpp
+++ b/llvm/lib/CodeGen/MachineCombiner.cpp
@@ -336,7 +336,7 @@
     auto &MI = *BlockIter++;
 
     DEBUG(dbgs() << "INSTR "; MI.dump(); dbgs() << "\n";);
-    SmallVector<MachineCombinerPattern::MC_PATTERN, 16> Patterns;
+    SmallVector<MachineCombinerPattern, 16> Patterns;
     // The motivating example is:
     //
     //     MUL  Other        MUL_op1 MUL_op2  Other