[TargetLowering] Add NegatibleCost enum for isNegatibleForFree return codes

The isNegatibleForFree/getNegatedExpression methods currently rely on a raw char value to indicate whether a negation is beneficial or not.

This patch replaces the char return value with an NegatibleCost enum to more clearly demonstrate what is implied.

It also renames isNegatibleForFree to getNegatibleCost to more accurately reflect whats going on.

Differential Revision: https://reviews.llvm.org/D74221