[X86] combineConstantPoolLoads - correctly merge constant pool loads by pointer and chain (#139575)

We were merging with a larger constant pool load if it didn't have any
chain dependencies (and implicitly assuming all uses were on the vector
value), instead we should flip this, explicitly check for uses of the
vector value and merge the memory chain dependencies with
makeEquivalentMemoryOrdering.

As these are constant pool loads we shouldn't expect any changes here,
but we should be consistent with how we merge/reuse loads - an upcoming
patch for other loads will do the same as we will see changes there.

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -53035,7 +53035,7 @@ static SDValue combineConstantPoolLoads(SDNode *N, const SDLoc &dl,
         (User->getOpcode() == X86ISD::SUBV_BROADCAST_LOAD ||
          User->getOpcode() == X86ISD::VBROADCAST_LOAD ||
          ISD::isNormalLoad(User)) &&
-        UserLd->getChain() == Chain && !User->hasAnyUseOfValue(1) &&
+        UserLd->getChain() == Chain && User->hasAnyUseOfValue(0) &&
         User->getValueSizeInBits(0).getFixedValue() >
             RegVT.getFixedSizeInBits()) {
       EVT UserVT = User->getValueType(0);
@@ -53057,6 +53057,7 @@ static SDValue combineConstantPoolLoads(SDNode *N, const SDLoc &dl,
               getTargetConstantBitsFromNode(SDValue(User, 0), NumBits,
                                             UserUndefs, UserBits)) {
             if (MatchingBits(Undefs, UserUndefs, Bits, UserBits)) {
+              DAG.makeEquivalentMemoryOrdering(SDValue(N, 1), SDValue(User, 1));
               SDValue Extract = extractSubVector(SDValue(User, 0), 0, DAG, dl,
                                                  RegVT.getSizeInBits());
               Extract = DAG.getBitcast(RegVT, Extract);