[AArch64][SVE2] Combine trunc+add+lsr to rshrnb

The example sequence

  add z0.h, z0.h, #32
  lsr z0.h, #6
  st1b z0.h, x1

can be replaced with

  rshrnb z0.b, #6
  st1b z0.h, x1

As the top half of the destination elements are truncated.

In similar fashion,

  add z0.s, z0.s, #32
  lsr z1.s, z1.s, #6
  add z1.s, z1.s, #32
  lsr z0.s, z0.s, #6
  uzp1 z0.h, z0.h, z1.h

Can be replaced with

  rshrnb z1.h, z1.s, #6
  rshrnb z0.h, z0.s, #6
  uzp1 z0.h, z0.h, z1.h

Differential Revision: https://reviews.llvm.org/D155299