[DAGCombine] Remove the getNegatibleCost to avoid the out of sync with getNegatedExpression

We have the getNegatibleCost/getNegatedExpression to evaluate the cost and negate the expression.
However, during negating the expression, the cost might change as we are changing the DAG,
and then, hit the assertion if we negated the wrong expression as the cost is not trustful anymore.

This patch is target to remove the getNegatibleCost to avoid the out of sync with getNegatedExpression,
and check the cost during negating the expression. It also reduce the duplicated code between
getNegatibleCost and getNegatedExpression. And fix the crash for the test in D76638

Reviewed By: RKSimon, spatel

Differential Revision: https://reviews.llvm.org/D77319