[NFC][DAGCombine] Adding three helper functions and change the...

[NFC][DAGCombine] Adding three helper functions and change the getNegatedExpression to negateExpression

This is a NFC patch for D77319. The idea is to hide the getNegatibleCost inside the getNegatedExpression()
to have it return null if the cost is expensive, and add some helper function for easy to use. And
rename the old getNegatedExpression to negateExpression to avoid the semantic conflict.

Reviewed By: RKSimon

Differential revision: https://reviews.llvm.org/D78291