[NFC][DAGCombine] Change the value of NegatibleCost to make it align with the semantics

This is a minor NFC change to make the code more clear. We have the NegatibleCost that
has cheaper, neutral, and expensive. Typically, the smaller one means the less cost.
It is inverse for current implementation, which makes following code not easy to read.
If (CostX > CostY) negate(X)

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D77993