[SelectionDAG][X86] Fix the assertion failure in Release build after #91747 (#93434)

In #91747, we changed the SDNode from `X86ISD::SUB` (FROM) to
`X86ISD::CCMP`
(TO) in the DAGCombine. The value type of `X86ISD::SUB` can be `i8, i32`
while the value type of `X86ISD::CCMP` is i32. This breaks the
assumption
that the value type should match after the combine and triggers the
error

```
SelectionDAG.cpp:10942: void
llvm::SelectionDAG::transferDbgValues(llvm::SDValue, llvm::SDValue,
unsigned int, unsigned int, bool): Assertion `FromNode && ToNode &&
"Can't modify dbg values"' failed.
```

when running tests

llvm/test/CodeGen/X86/apx/ccmp.ll
llvm/test/CodeGen/X86/apx/ctest.ll

in Release build when LLVM_ENABLE_ASSERTIONS is on.

In this patch, we fix it by creating a merged value.