SUBVL is effectively disabled (a SUBVL for-loop of only one element). this
lines up in combination with all other "default is all zeros" behaviour.
-| Value | Mnemonic | xxx | Description |
+| Value | Mnemonic | Subvec | Description |
|-------|-----------|---------|------------------------|
-| 00 | `SUBVL=1` | default | Sub-vector length of 1 |
+| 00 | `SUBVL=1` | single | Sub-vector length of 1 |
| 01 | `SUBVL=2` | vec2 | Sub-vector length of 2 |
| 10 | `SUBVL=3` | vec3 | Sub-vector length of 3 |
| 11 | `SUBVL=4` | vec4 | Sub-vector length of 4 |
| Value | Mnemonic | Element `i` enabled if: |
|-------|----------|------------------------------|
-| 000 | ALWAYS | (Operation is not masked) |
+| 000 | ALWAYS | predicate effectively all 1s |
| 001 | 1 << R3 | `i == R3` |
| 010 | R3 | `R3 & (1 << i)` is non-zero |
| 011 | ~R3 | `R3 & (1 << i)` is zero |