[AArch64][SVE] Asm: Support for bit/byte reverse operations.

This patch adds the following instructions:

  RBIT      reverse bits within each active elemnt (predicated), e.g.
                rbit z0.d, p0/m, z1.d

            for 8, 16, 32 and 64 bit elements.

  REV       reverse order of elements in data/predicate vector
            (unpredicated), e.g.
                rev z0.d, z1.d
                rev p0.d, p1.d

            for 8, 16, 32 and 64 bit elements.

  REVB      reverse order of bytes within each active element, e.g.
                revb z0.d, p0/m, z1.d

            for 16, 32 and 64 bit elements.

  REVH      reverse order of 16-bit half-words within each active
            element, e.g.
                revh z0.d, p0/m, z1.d

            for 32 and 64 bit elements.

  REVW      reverse order of 32-bit words within each active element,
            e.g.
                revw z0.d, p0/m, z1.d

            for 64 bit elements.

llvm-svn: 337534