[mlir][Vector] Fold InsertStridedSliceOp of two splat with the same input to splat.

This patch folds InsertStridedSliceOp(SplatOp(X):src_type, SplatOp(X):dst_type) to SplatOp(X):dst_type.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D128891