[AArch64][SVE] Asm: Print indexed element 0 as FPR.

Print the first indexed element as a FP register, for example:

  mov z0.d, z1.d[0]

Is now printed as:

  mov z0.d, d1

Next to printing, this patch also adds aliases to parse 'mov z0.d, d1'.

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

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D47571

llvm-svn: 333872