[DAGCombiner] Use SmallDenseMap (NFC) (#79681)

The use of SmallDenseMap saves 0.48% of heap allocations during the
compilation of a large preprocessed file, namely X86ISelLowering.cpp,
for the X86 target.  During the experiment, the maximum size of
WorklistMap was 24 or less 74% of the time.  (Note that DenseMap has
the maximum occupancy rate of 3/4.)