[GreedyRA ORE] Add Cost of spill locations into remark

Cost of spill location is computed basing on relative branch frequency
where corresponding spill/reload/copy are located.

While the number itself is highly depends on incoming IR,
the total cost can be used when do some changes in RA.

Reviewers: reames, MatzeB, anemet, thegameg
Reviewed By: reames
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D100020