[AArch64][SVE] Asm: Support for CNT(B|H|W|D) and CNTP instructions.

This patch adds support for the following instructions:

  CNTB CNTH - Determine the number of active elements implied by
  CNTW CNTD   the named predicate constant, multiplied by an
              immediate, e.g.

                cnth x0, vl8, #16

  CNTP      - Count active predicate elements, e.g.
                cntp  x0, p0, p1.b

              counts the number of active elements in p1, predicated
              by p0, and stores the result in x0.

llvm-svn: 336552