In order to align with `svext` and NEON `vext`/`vextq`, this patch
changes immediate argument in `svextq` such that it refers to elements
of the size of those of the source vector, rather than bytes. The [spec
for this
intrinsic](https://github.com/ARM-software/acle/blob/main/main/acle.md#extq)
is ambiguous about the meaning of this argument, this issue was raised
after there was a differing interpretation for it from the implementers
of the ACLE in GCC.
For example (with our current implementation):
`svextq_f64(zn_f64, zm_f64, 1)` would, for each 128-bit segment of
`zn_f64,` concatenate the highest 15 bytes of this segment with the
first byte of the corresponding segment of `zm_f64`.
After this patch, the behavior of `svextq_f64(zn_f64, zm_f64, 1)` would
be, for each 128-bit vector segment of `zn_f64`, to concatenate the
higher doubleword of this segment with the lower doubleword of the
corresponding segment of `zm_f64`.
The range of the immediate argument in `svextq` would be modified such
that it is:
- [0,15] for `svextq_{s8,u8}`
- [0,7] for `svextq_{s16,u16,f16,bf16}`
- [0,3] for `svextq_{s32,u32,f32}`
- [0,1] for `svextq_{s64,u64,f64}`
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