Ends a split phase norm computation.
x - the first vector
ntype - norm type, one of NORM_1, NORM_2, NORM_MAX, NORM_1_AND_2
result - where the result will go
Each call to VecNormBegin() should be paired with a call to VecNormEnd().
The x vector is not allowed to be NULL, otherwise the vector would not have its correctly cached norm value