Added a generalization of isBytewiseValue that is called
isSplatValue. It takes the requested splat size (in bits)
as an additional argument.
The old isBytewiseValue remains in the code as a shortcut
for isSplatValue(V, 8).
This refactoring can be useful both when looking for non
bytewise splats, but also for out-of-tree targets that
for example implement 16-bit bytes (as they now can
override the byte size used in isBytewiseValue more easily).