X86InstrInfo: Optimize more combinations of SUB+CMP

`X86InstrInfo::optimizeCompareInstr` would only optimize a `SUB`
followed by a `CMP` in `isRedundantFlagInstr`. This extends the code to
also look for other combinations like `CMP`+`CMP`, `TEST`+`TEST`, `SUB
x,0`+`TEST`.

- Change `isRedundantFlagInstr` to run `analyzeCompareInstr` on the
  candidate instruction and compare the results. This normalizes things
  and gives consistent results for various comparisons (`CMP x, y`,
  `SUB x, y`) and immediate cases (`TEST x, x`, `SUB x, 0`,
  `CMP x, 0`...).
- Turn `isRedundantFlagInstr` into a member function so it can call
  `analyzeCompare`.  - We now also check `isRedundantFlagInstr` even if
  `IsCmpZero` is true, since we now have cases like `TEST`+`TEST`.

Differential Revision: https://reviews.llvm.org/D110865