Missing cases in testing for `shuffle` and `shuffle2`

The CTS doesn't test the following cases:
- Mask values with bits that must be ignored set:

>For shuffle, only theilogb(2m-1) least significant bits of each mask element are considered. For shuffle2, only the ilogb(2m-1)+1 least significant bits of each mask element are considered. Other bits in the mask shall be ignored.

- Non-constant mask values

Start by locating the CTS tests for shuffle and shuffle2. Add coverage for masks with ignored bits set and for non-constant mask values, then run the relevant shuffle and shuffle2 tests to confirm these cases pass.

Written by the indexing model from the issue text.