[llvm] [X86] Fold `(icmp ult (add x,-C),2)` -> `(or (icmp eq X,C), (icmp eq X,C+1))` for Vectors (PR #84104)

[Simon Pilgrim via llvm-commits]

================
@@ -53408,6 +53409,64 @@ static SDValue combineSetCC(SDNode *N, SelectionDAG &DAG,
           truncateAVX512SetCCNoBWI(VT, OpVT, LHS, RHS, CC, DL, DAG, Subtarget))
     return R;
 
+  // In the middle end transforms:
+  //    `(or (icmp eq X, C), (icmp eq X, C+1))`
+  //        -> `(icmp ult (add x, -C), 2)`
+  // Likewise inverted cases with `ugt`.
+  //
+  // Since x86, pre avx512, doesn't have unsigned vector compares, this results
+  // in worse codegen. So, undo the middle-end transform and go back to `(or
+  // (icmp eq), (icmp eq))` form.
+  //
+  // NB: We don't handle the similiar simplication of `(and (icmp ne), (icmp
+  // ne))` as it doesn't end up instruction positive.
+  // TODO: We might want to do this for avx512 as well if we `sext` the result.
+  if (VT.isVector() && OpVT.isVector() && OpVT.isInteger() &&
+      ISD::isUnsignedIntSetCC(CC) && LHS.getOpcode() == ISD::ADD &&
+      !Subtarget.hasAVX512() && LHS.hasOneUse()) {
+
+    APInt CmpC;
+    SDValue AddC = LHS.getOperand(1);
+    if (ISD::isConstantSplatVector(RHS.getNode(), CmpC) &&
+        DAG.isConstantIntBuildVectorOrConstantInt(AddC)) {
+      // See which form we have depending on the constant/condition.
+      SDValue C0 = SDValue();
+      SDValue C1 = SDValue();
+
+      // If we had `(add x, -1)` and can lower with `umin`, don't transform as
+      // we will end up generating an additional constant. Keeping in the
+      // current form has a slight latency cost, but it probably worth saving a
+      // constant.
+      if (ISD::isConstantSplatVectorAllOnes(AddC.getNode()) &&
+          DAG.getTargetLoweringInfo().isOperationLegal(ISD::UMIN, OpVT)) {
----------------
RKSimon wrote:

Please can you add avx1 test coverage and some 256-bit vectors - ISD::UMIN 256-bit will be custom so we need to see the effect

https://github.com/llvm/llvm-project/pull/84104