[AArch64][SVE] Asm: Instructions to perform serialized operations.

The instructions added in this patch permit active elements within
a vector to be processed sequentially without unpacking the vector.

  PFIRST      Set the first active element to true.
  PNEXT       Find next active element in predicate.
  CTERMEQ     Compare and terminate loop when equal.
  CTERMNE     Compare and terminate loop when not equal.

llvm-svn: 338210