[GreedyRA ORE] Extract computeNumberOfSplillsReloads to use in different places. NFC.

Extract one basic block handling to introduce stat computation for method scope.

Reviewers: reames, MatzeB, anemet, thegameg
Reviewed By: reames, thegameg
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D100013