[LoopFusion] Simplifying the legality checks (#171889)

Considering that the current loop fusion only supports adjacent loops,
we are able to simplify the checks in this pass. By removing
`isControlFlowEquivalent` check, this patch fixes multiple issues
including #166560, #166535, #165031, #80301 and #168263.

Now only the sequential/adjacent candidates are collected in the same
list. This patch is the implementation of approach 2 discussed in post
#171207.

Author: 1997alireza

llvm/include/llvm/Transforms/Utils/CodeMoverUtils.h

@@ -27,22 +27,6 @@ class DominatorTree;
 class Instruction;
 class PostDominatorTree;
 
-/// Return true if \p I0 and \p I1 are control flow equivalent.
-/// Two instructions are control flow equivalent if their basic blocks are
-/// control flow equivalent.
-LLVM_ABI bool isControlFlowEquivalent(const Instruction &I0,
-                                      const Instruction &I1,
-                                      const DominatorTree &DT,
-                                      const PostDominatorTree &PDT);
-
-/// Return true if \p BB0 and \p BB1 are control flow equivalent.
-/// Two basic blocks are control flow equivalent if when one executes, the other
-/// is guaranteed to execute.
-LLVM_ABI bool isControlFlowEquivalent(const BasicBlock &BB0,
-                                      const BasicBlock &BB1,
-                                      const DominatorTree &DT,
-                                      const PostDominatorTree &PDT);
-
 /// Return true if \p I can be safely moved before \p InsertPoint.
 LLVM_ABI bool isSafeToMoveBefore(Instruction &I, Instruction &InsertPoint,
                                  DominatorTree &DT,