[AArch64][SME] Update tile slice index offset

[AArch64][SME] Update tile slice index offset

Changes in architecture revision 00eac1:
  * Tile slice index offset no longer prefixed with '#'.
  * The syntax for 128-bit (.Q) ZA tile slice accesses must now include
    an explicit zero index.

The reference can be found here:
https://developer.arm.com/documentation/ddi0602/2021-09

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D111212