[LSR] Early exit for RateFormula when it is already losing. NFC

[LSR] Early exit for RateFormula when it is already losing. NFC

This patch does not effect any behavior of the current code.

The codebase implicitly implies that `Cost::RateFormula` is only called
when the `Cost` is not in losing status, or else there may be possible
to trigger the assertion of `Cost::isValid`.

The intention here is to prevent mis-use where future development
allow `Cost` that is already loser to call `Cost::RateFormula` - Early
exit when `Cost` is already losing.

Reviewed By: Meinersbur, #loopoptwg

Differential Revision: https://reviews.llvm.org/D125670