[AArch64][SVE] Asm: Support for FDUP_ZI (copy fp immediate) instruction.

Unpredicated copy of floating-point 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/D47482

llvm-svn: 333744