[mlir][sparse] cleanup small vector constant hints

Following advise from

https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h

This revision removes the size hints from SmallVector (unless we are
certain of the resulting number of elements). Also, this replaces
SmallVector references with SmallVectorImpl references.

Reviewed By: Peiming

Differential Revision: https://reviews.llvm.org/D138063