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