[RS4GC] Extract rematerilazable candidate search. NFC.

Finding re-materialization chain for derived pointer does not depend on
call site. To avoid this finding for each call site it can be extracted in
a separate routine.

Reviewers: reames, dantrushin
Reviewed By: reames
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D118676