Fix #12937 FP knownConditionTrueFalse for flags linked by condition

[chrchr-github]

@pfultz2 Is there a better way to handle this?

[chrchr-github]

It looks like only operator conditions are flipped correctly in an else block.
This seems to work as well:

const bool isOp = (condTok->isComparisonOp() || condTok->tokType() == Token::eLogicalOp);
const bool is1 = isOp || astIsBool(condTok);
[...]
                    if (is1) {
                        OppositeExpressionAnalyzer a2(true, condTok2, makeConditionValue(isOp, condTok2, false, false, settings), settings);
                        valueFlowGenericForward(startTok, startTok->link(), a2, tokenlist, errorLogger, settings);
                    }