[IR] Simplify Use::swap. NFCI.

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.

Differential Revision: https://reviews.llvm.org/D80116

Author: jayfoad

llvm/lib/IR/Use.cpp

@@ -17,24 +17,17 @@ 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 {

llvm/test/Transforms/LoopReroll/nonconst_lb.ll

@@ -48,15 +48,15 @@ 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