[AArch64][SVE] Asm: Support for indexed DUP instructions.

Unpredicated copy of indexed SVE element to SVE vector,
along with MOV-aliases.

For example:

  dup     z0.h, z1.h[0]

duplicates the first 16-bit element from z1 to all elements in
the result vector z0.

Reviewers: rengolin, fhahn, samparker, SjoerdMeijer, javed.absar

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D47570

llvm-svn: 333871