Revert "[X86] Use X86ISD::SUB instead of X86ISD::CMP in some places."

This reverts commit 8413116b.

this seems to be causing crashes while compiling ncurses.
```
$ ./bin/llc bugpoint-reduced-simplified.ll
LLVM ERROR: Cannot emit physreg copy instruction
```

Here are the crashers: https://gist.github.com/kadircet/918f5bb97a2afe048cb875490edba46e

executing with an llc compiled at 904d54de works fine.