Fixed a bug about disassembling AArch64 post-index load/store single element instructions.

ie. echo "0x00 0x04 0x80 0x0d" | ../bin/llvm-mc -triple=aarch64 -mattr=+neon -disassemble
    echo "0x00 0x00 0x80 0x0d" | ../bin/llvm-mc -triple=aarch64 -mattr=+neon -disassemble
will be disassembled into the same instruction st1 {v0b}[0], [x0], x0.

llvm-svn: 195591