[AArch64][SVE][Fixed length] Fix div miscompile

The prior code worked before SVE DIV was enabled 128 bit vectors.
With 128 bit vectors, when run on a 256 bit machine, it would split and
do a signed unpack, but this resulted in one full vector and one empty
vector with a half-sized predicate. The effect was that only half the
elements were treated correctly.

The fix is to bisect the vector, sign extend, do the division, truncate
and then concat.

Fixes #59357.

Differential Revision: https://reviews.llvm.org/D139618