[AArch64][SVE] Fix bug in lowering of fixed-length integer vector divides

The function AArch64TargetLowering::LowerFixedLengthVectorIntDivideToSVE
previously assumed the operands were full vectors, but this is not
always true. This function would produce bogus if the division operands
are not full vectors, resulting in miscompiles when dividing 8-bit or
16-bit vectors.

The fix is to perform an extend + div + truncate for non-full vectors,
instead of the usual unpacking and unzipping logic. This is an additive
change which reduces the non-full integer vector divisions to a pattern
recognised by the existing lowering logic.

For future reference, an example of code that would miscompile before
this patch is below:

     1  int8_t foo(unsigned N, int8_t *a, int8_t *b, int8_t *c) {
     2      int8_t result = 0;
     3      for (int i = 0; i < N; ++i) {
     4          result += (a[i] / b[i]) / c[i];
     5      }
     6      return result;
     7  }

Differential Revision: https://reviews.llvm.org/D100370