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

As far as I can tell if getIndexedAddressParts received an ISD::SUB, the
constant would be negated. So `IsInc` should be set to true since the
SUB was effectively converted to ADD. This means we should never use
PRE_DEC/POST_DEC.

No tests are affected because DAGCombine aggressively turns SUB with
constant into ADD so no lit test has a SUB reach getIndexedAddressParts.