[TableGen] Change std::vector to SmallVector

The size of VTList that is pushed into this container is usually 1, but
often 6 or 7. Change the vector to SmallVector to eliminate frequent
mallocs.  This happens hundreds of thousands of times in each tablegen
execution during the LLVM/clang build.

https://reviews.llvm.org/D83849

llvm/utils/TableGen/CodeGenDAGPatterns.h

@@ -190,7 +190,7 @@ private:
 
 struct TypeSetByHwMode : public InfoByHwMode<MachineValueTypeSet> {
   using SetType = MachineValueTypeSet;
-  std::vector<unsigned> AddrSpaces;
+  SmallVector<unsigned, 16> AddrSpaces;
 
   TypeSetByHwMode() = default;
   TypeSetByHwMode(const TypeSetByHwMode &VTS) = default;