[CodeMoverUtils] Enhance CodeMoverUtils to sink an entire BB (#87857)

When moving an entire basic block after `InsertPoint`, currently we
check each instruction whether their users are dominated by
`InsertPoint`, however, this can be improved such that even a user is
not dominated by `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. Please refer to the added functionality in test case
`llvm/unittests/Transforms/Utils/CodeMoverUtilsTest.cpp` that was not
supported without this patch.

Author: CongzheUalberta

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));
       });
 }