Currently there are match rules named as xxx_masked_partial, which are
expected to work on masked vector operations when the vector size is not
full size of hardware vector reg width, i.e. partial vector. Those rules
will make sure the given masked (predicate) high bits are clear within
vector width. Actually, if we can guarantee the input predicate high
bits are already clear with vector width, we don't need to re-do the
clear work before use.