[mlir][vector] Add support for scalable vectors in `trimLeadingOneDims`

This patch updates one specific hook in "VectorDropLeadUnitDim.cpp" to
make sure that "scalable dims" are handled correctly. While this change
affects multiple patterns, I am only adding one regression tests that
captures one specific case that affects me right now.

I am also adding Vector dialect to the list of dependencies of
`-test-vector-to-vector-lowering`. Otherwise my test case won't work as
a standalone test.

Differential Revision: https://reviews.llvm.org/D157993