[AArch64][SVE] Asm: Support for CPY immediate instructions

Predicated copy of possibly shifted immediate value into SVE
vector, along with MOV-aliases.

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

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D47517

llvm-svn: 333868