[llvm] [RISCV] Remove incomplete PRE_DEC/POST_DEC code for XTHeadMemIdx. (PR #76922)

[Philipp Tomsich via llvm-commits]

https://github.com/ptomsich approved this pull request.

Reviewed and LGTM.
I didn't rerun tests, as that was already done by Craig.

https://github.com/llvm/llvm-project/pull/76922