X86InstrInfo: Support immediates that are +1/-1 different in optimizeCompareInstr

This is a re-commit of e2c7ee07 which
was reverted in a2a58d91. This includes
a fix to consistently check for EFLAGS being live-out. See phabricator
review.

Original Summary:

This extends `optimizeCompareInstr` to re-use previous comparison
results if the previous comparison was with an immediate that was 1
bigger or smaller. Example:

    CMP x, 13
    ...
    CMP x, 12   ; can be removed if we change the SETg
    SETg ...    ; x > 12  changed to `SETge` (x >= 13) removing CMP

Motivation: This often happens because SelectionDAG canonicalization
tends to add/subtract 1 often when optimizing for fallthrough blocks.
Example for `x > C` the fallthrough optimization switches true/false
blocks with `!(x > C)` --> `x <= C` and canonicalization turns this into
`x < C + 1`.

Differential Revision: https://reviews.llvm.org/D110867