[CodeMoverUtils] Enhance CodeMoverUtils to sink entire BB

When moving an entire basic block BB after InsertPoint, currently
we check for all instructions whether their users are dominated by
InsertPoint, however, this can be improved such that even a user
is not dominated InsertPoint, as long as it appears as a subsequent
instruction in the same BB, it is safe to move.

This patch is similar to commit 751be2a
that enhanced hoisting an entire BB, and this patch enhances sinking an
entire BB.

Author: Congzhe Cao

llvm/lib/Transforms/Utils/CodeMoverUtils.cpp

@@ -336,9 +336,22 @@ bool llvm::isSafeToMoveBefore(Instruction &I, Instruction &InsertPoint,
 
   if (isReachedBefore(&I, &InsertPoint, &DT, PDT))
     for (const Use &U : I.uses())
-      if (auto *UserInst = dyn_cast<Instruction>(U.getUser()))
-        if (UserInst != &InsertPoint && !DT.dominates(&InsertPoint, U))
+      if (auto *UserInst = dyn_cast<Instruction>(U.getUser())) {
+        // If InsertPoint is in a BB that comes after I, then we cannot move if
+        // I is used in the terminator of the current BB.
+        if (I.getParent() == InsertPoint.getParent() &&
+            UserInst == I.getParent()->getTerminator())
           return false;
+        if (UserInst != &InsertPoint && !DT.dominates(&InsertPoint, U)) {
+          // If UserInst is an instruction that appears later in the same BB as
+          // I, then it is okay to move since I will still be available when
+          // UserInst is executed.
+          if (CheckForEntireBlock && I.getParent() == UserInst->getParent() &&
+              DT.dominates(&I, UserInst))
+            continue;
+          return false;
+        }
+      }
   if (isReachedBefore(&InsertPoint, &I, &DT, PDT))
     for (const Value *Op : I.operands())
       if (auto *OpInst = dyn_cast<Instruction>(Op)) {

llvm/unittests/Transforms/Utils/CodeMoverUtilsTest.cpp

@@ -673,6 +673,11 @@ TEST(CodeMoverUtils, IsSafeToMoveTest4) {
         // Can move as %add2 and %sub2 are control flow equivalent,
         // although %add2 does not strictly dominate %sub2.
         EXPECT_TRUE(isSafeToMoveBefore(*SubInst2, *AddInst2, DT, &PDT, &DI));
+
+        BasicBlock *BB0 = getBasicBlockByName(F, "if.then.first");
+        BasicBlock *BB1 = getBasicBlockByName(F, "if.then.second");
+        EXPECT_TRUE(
+            isSafeToMoveBefore(*BB0, *BB1->getTerminator(), DT, &PDT, &DI));
       });
 }