[llvm] [InlineOrder] fix the calculation of Cost for CostBenefitPriority (PR #86630)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

The patch looks good.  Do you happen to have a test case that tripped on `assert(isVariable() ...` in `getCost()`?  If so, we should incorporate that into this PR.

https://github.com/llvm/llvm-project/pull/86630