X86InstrInfo: Refactor and cleanup optimizeCompareInstr

This changes the first part of `optimizeCompareInstr` being split into a
loop with a forward scan for cases that re-use zero flags from a
producer in case of compare with zero and a backward scan for finding an
instruction equivalent to a compare.

The code now uses a single backward scan searching for the next
instructions that reads or writes EFLAGS.

Also:
- Add comments giving examples for the 3 cases handled.
- Check `MI` which contains the result of the zero-compare cases,
  instead of re-checking `IsCmpZero`.
- Tweak coding style in some loops.
- Add new MIR based tests that test the optimization in isolation.

This also removes a check for flag readers in situations like this:
```
= SUB32rr %0, %1, implicit-def $eflags
...  we no longer stop when there are $eflag users here
CMP32rr %0, %1   ; will be removed
...
```

Differential Revision: https://reviews.llvm.org/D110857