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

[Wang Pengcheng via llvm-commits]

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@@ -19288,7 +19287,8 @@ bool RISCVTargetLowering::getIndexedAddressParts(SDNode *Op, SDValue &Base,
     if (!isLegalIndexedOffset)
       return false;
 
-    IsInc = (Op->getOpcode() == ISD::ADD);
+    // Constant for SUB was negated earlier.
+    IsInc = true;
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wangpc-pp wrote:

If `IsInc` is always true, then we don't need this parameter in `getIndexedAddressParts`.

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