[AArch64] Fix operand definitions of XPACI/XPACD

The operand to these instructions is both input and output.

These are not yet emitted by the compiler and the assembler already
works fine, so can't test in this patch.  But D75044 will use XPACI
and provide test coverage for this patch as well.

Differential Revision: https://reviews.llvm.org/D84298