[VectorUtils] rename scaleShuffleMask to narrowShuffleMaskElts; NFC

[VectorUtils] rename scaleShuffleMask to narrowShuffleMaskElts; NFC

As proposed in D77881, we'll have the related widening operation,
so this name becomes too vague.

While here, change the function signature to take an 'int' rather
than 'size_t' for the scaling factor, add an assert for overflow of
32-bits, and improve the documentation comments.