[AArch64][GlobalISel] Fix constraining LDXPX intrinsic selection.

Causes a fallback because of lack of regclasses on vregs, unless its without
asserts, where we end up crashing later in codegen.

llvm/lib/Target/AArch64/GISel/AArch64InstructionSelector.cpp

@@ -5042,6 +5042,7 @@ bool AArch64InstructionSelector::selectIntrinsicWithSideEffects(
         {I.getOperand(0).getReg(), I.getOperand(1).getReg()},
         {I.getOperand(3)});
     NewI.cloneMemRefs(I);
+    constrainSelectedInstRegOperands(*NewI, TII, TRI, RBI);
     break;
   }
   case Intrinsic::trap: