[AArch64][SVE] Asm: Support for DUPM (masked immediate) instruction.

Unpredicated copy of repeating immediate pattern to SVE vector, along
with MOV-aliases.

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

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D47328

llvm-svn: 333731