[InstCombine] Fix replace select with Phis when branch has the same labels

```
define i32 @test(i1 %cond) {
entry:
  br i1 %cond, label %exit, label %exit
exit:
  %result = select i1 %cond, i32 123, i32 456
  ret i32 %result
}
```
In this test, after applying transformation of replacing select with Phis,
the result will be:

```
define i32 @test(i1 %cond) {
entry:
  br i1 %cond, label %exit, label %exit
exit:
  %result = i32 phi [123, %exit], [123, %exit]
  ret i32 %result
}
```
That is, select is transformed into an invalid Phi, which will then be
reduced to 123 and the second value will be lost. But it is worth
noting that this problem will arise only if select is in the InstCombine
worklist will be before the branch. Otherwise, InstCombine will replace
the branch condition with false and transformation will not be applied.

The fix is to check the target labels in the branch condition for equality.

Patch By: Kirill Polushin
Differential Revision: https://reviews.llvm.org/D84003
Reviewed By: mkazantsev

lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -2469,6 +2469,10 @@ static Instruction *foldSelectToPhiImpl(SelectInst &Sel, BasicBlock *BB,
   } else
     return nullptr;
 
+  // Make sure the branches are actually different.
+  if (TrueSucc == FalseSucc)
+    return nullptr;
+
   // We want to replace select %cond, %a, %b with a phi that takes value %a
   // for all incoming edges that are dominated by condition `%cond == true`,
   // and value %b for edges dominated by condition `%cond == false`. If %a