[mlir][sparse] Fixes bug in VarSet ctor

Previously, the commented out code in the `DimLvlMap` ctor would result in `VarSet::add` raising an OOB error; which should be impossible because the ctor asserted `DimLvlMap::isWF` which ensures that all variables occuring in the map are within bounds for the ranks.

The root cause of that bug was the `VarSet` ctor using `SmallBitVector::reserve` which does not actually change the size of the bitvectors (hence the subsequent OOB).  This is corrected by using any of `SmallBitVector::resize`, the move-ctor, or the copy-ctor.  Since the default-initialized bitvectors being modified/overwritten have size zero, there shouldn't be any significant performance difference between these three implementations.

Reviewed By: Peiming

Differential Revision: https://reviews.llvm.org/D155999