[IR] Simplify Use::swap. NFCI.

[foad]

Summary

The new implementation makes it clear that there are exactly two
conditional stores (after the initial no-op optimization). By contrast
the old implementation had seven conditionals, some hidden inside other
functions.

This commit can change the order of operands in operand lists, hence the
tweak to one test case.

[ekatz]

I like it! Great job!

[ekatz]

I just wonder if there is anyone counting on the current ordering of uses.
Although, I think they shouldn't...

llvm/lib/IR/Use.cpp

 #include <new>
 
 namespace llvm {
 
 void Use::swap(Use &RHS) {
   if (Val == RHS.Val)
     return;
 
-  if (Val)
-    removeFromList();
-
-  Value *OldVal = Val;
-  if (RHS.Val) {
-    RHS.removeFromList();
-    Val = RHS.Val;
-    Val->addUse(*this);
-  } else {
-    Val = nullptr;
-  }
-
-  if (OldVal) {
-    RHS.Val = OldVal;
-    RHS.Val->addUse(RHS);
-  } else {
-    RHS.Val = nullptr;
-  }
+  std::swap(Val, RHS.Val);
+  std::swap(Next, RHS.Next);
+  std::swap(Prev, RHS.Prev);
+
+  *Prev = this;
+  if (Next)
+    Next->Prev = &Next;
+
+  *RHS.Prev = &RHS;
+  if (RHS.Next)
+    RHS.Next->Prev = &RHS.Next;
 }
 
 unsigned Use::getOperandNo() const {
   return this - getUser()->op_begin();
 }
 
 void Use::zap(Use *Start, const Use *Stop, bool del) {
   while (Start != Stop)
     (--Stop)->~Use();
   if (del)
     ::operator delete(Start);
 }
 
 } // namespace llvm

llvm/test/Transforms/LoopReroll/nonconst_lb.ll

for.body:                                         ; preds = %entry, %for.body
   %add18 = add nsw i32 %i.035, 4
   %cmp = icmp slt i32 %add18, %n
   br i1 %cmp, label %for.body, label %for.end
 
 for.end:                                          ; preds = %for.body, %entry
   ret void
 }
 ; CHECK-LABEL: @foo
-; CHECK: for.body.preheader:                               ; preds = %entry
+; CHECK: for.body.preheader:
 ; CHECK:   %0 = add i32 %n, -1
 ; CHECK:   %1 = sub i32 %0, %m
 ; CHECK:   %2 = lshr i32 %1, 2
 ; CHECK:   %3 = shl nuw i32 %2, 2
 ; CHECK:   %4 = add i32 %3, 3
 ; CHECK:   br label %for.body
 
-; CHECK: for.body:                                         ; preds = %for.body, %for.body.preheader
+; CHECK: for.body:
 ; CHECK:   %indvar = phi i32 [ 0, %for.body.preheader ], [ %indvar.next, %for.body ]
 ; CHECK:   %5 = add i32 %m, %indvar
 ; CHECK:   %arrayidx = getelementptr inbounds i32, i32* %B, i32 %5
 ; CHECK:   %6 = load i32, i32* %arrayidx, align 4
 ; CHECK:   %mul = shl nsw i32 %6, 2
 ; CHECK:   %arrayidx2 = getelementptr inbounds i32, i32* %A, i32 %5
 ; CHECK:   store i32 %mul, i32* %arrayidx2, align 4
 ; CHECK:   %indvar.next = add i32 %indvar, 1