[AArch64][SVE] Asm: Support for FCPY immediate instructions.

Predicated copy of floating-point immediate value to SVE vector,
along with MOV-aliases.

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

Reviewed By: javed.absar

Differential Revision: https://reviews.llvm.org/D47518

llvm-svn: 333869