[TableGen] Add a DenseMapInfo for MachineValueType.

No functional change, just adding a DenseMapInfo and tombstone
value so that MVT's can be put into a DenseMap / DenseSet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313782 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed