`xGBMV` performs the matrix-vector multiply-add operation of the form `y := alpha*op( A )*x + beta*y`, where `op( X )` is one of `op( A ) = A` or `op( A ) = A'`.

`ALPHA` and `BETA` are scalars, and `A` is an `M x N` banded matrix, with `KL` sub-diagonals and `KU` super-diagonals.

`X` is a vector of size `(1 + ( N - 1 ) * abs(INCX))` when argument `TRANS` is `NoTrans`, and `(1 + ( M - 1 ) * abs(INCX))` otherwise. `Y` is a vector of size `(1 + ( M - 1 ) * abs(INCY))` when argument `TRANS` is `NoTrans`, and `(1 + ( N - 1 ) * abs(INCY))` otherwise.