[MachineVerifier] Use Register instead of unsigned for DenseSet key. NFC (#128234)

Author: topperc

llvm/lib/CodeGen/MachineVerifier.cpp

@@ -3228,7 +3228,7 @@ struct VRegFilter {
   // worst-case memory usage within 2x of figures determined empirically for
   // "all Dense" scenario in such worst-by-execution-time cases.
   BitVector Sparse;
-  DenseSet<unsigned> Dense;
+  DenseSet<Register> Dense;
 };
 
 // Implements both a transfer function and a (binary, in-place) join operator