[AArch64][SVE] Asm: Support for UDOT/SDOT instructions.

The signed/unsigned DOT instructions perform a dot-product on
quadtuplets from two source vectors and accumulate the result in
the destination register. The instructions come in two forms:

Vector form, e.g.
  sdot  z0.s, z1.b, z2.b     - signed dot product on four 8-bit quad-tuplets,
                               accumulating results in 32-bit elements.

  udot  z0.d, z1.h, z2.h     - unsigned dot product on four 16-bit quad-tuplets,
                               accumulating results in 64-bit elements.

Indexed form, e.g.
  sdot  z0.s, z1.b, z2.b[3]  - signed dot product on four 8-bit quad-tuplets
                               with specified quadtuplet from second
                               source vector, accumulating results in 32-bit
                               elements.
  udot  z0.d, z1.h, z2.h[1]  - dot product on four 16-bit quad-tuplets
                               with specified quadtuplet from second
                               source vector, accumulating results in 64-bit
                               elements.

llvm-svn: 337372