[mlir] StandardToLLVM: clean up conversion patterns for vector operations

Summary:
Provide a public VectorConvertToLLVMPattern utility class to implement
conversions with automatic unrolling of operation on multidimensional vectors
to lists of operations on single-dimensional vectors when lowering to the LLVM
dialect. Drop the template-based check on the number of operands since the
actual implementation does not depend on the operand number anymore. This check
only creates spurious concepts (UnaryOpLowering, BinaryOpLowering, etc).

Differential Revision: https://reviews.llvm.org/D76865