[mlir][vector] Enable CastAwayElementwiseLeadingOneDim for scalable vec

This patch effectively enables the CastAwayElementwiseLeadingOneDim
rewrite pattern for scalable vectors. To this end,
`ExtractOp::inferReturnTypes` is updated so that scalable dimensions are
correctly recognised.

The change to ExtractOp will likely make also other conversion patterns
valid for scalable vectors, but this patch focuses on just one case.
Other conversion patterns will be enabled in the forthcoming patches.

Depends on D157993

Differential Revision: https://reviews.llvm.org/D158335