Fix InstructionComparer for ref_element_addr

InstructionComparer should only check for equality on additional state,
not operands. Fix visitRefElementAddr which was not using this common
pattern.

lib/SIL/IR/SILInstruction.cpp

@@ -544,8 +544,6 @@ namespace {
       auto *X = cast<RefElementAddrInst>(LHS);
       if (X->getField() != RHS->getField())
         return false;
-      if (X->getOperand() != RHS->getOperand())
-        return false;
       return true;
     }