[GreedyRA ORE] Extract stats in RAGreedyStats struct. NFC.

Combine all collected stats into separate struct RAGreedyStats
with add and report methods.

The motivation is to extend the number of statistics to capture and instead of
adding new parameters, just combine all of them into one structure.
Additionally I plan to use report from different places in future to report data
for function as well.

Reviewers: reames, MatzeB, anemet, thegameg
Reviewed By: thegameg
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D100012