[TableGen] Use MachineValueTypeSet in place of SmallSet.

MachineValueTypeSet is effectively a std::bitset<256>. This allows
us quickly insert into the set and check if a type is in the set.

llvm/utils/TableGen/CodeGenDAGPatterns.cpp

@@ -581,7 +581,7 @@ bool TypeInfer::EnforceVectorEltTypeIs(TypeSetByHwMode &Vec,
     Changed |= berase_if(E, isVector);  // Vector = !scalar
     assert(!V.empty() && !E.empty());
 
-    SmallSet<MVT,4> VT, ST;
+    MachineValueTypeSet VT, ST;
     // Collect element types from the "vector" set.
     for (MVT T : V)
       VT.insert(T.getVectorElementType());