[RS4GC] Make PointerToBase mapping be independent on call site. NFC.

PointerToBase is a mapping between potentially derived pointer to its base.
As soon as we are in SSA form if there is a base of derived pointer and it
is available at def of derived pointer, the same base will be available at any
point where derived pointer is alive.

So the mapping of derived pointer to base pointer is not a property
of a call site but the same on function level.

Reviewers: reames, yrouban
Reviewed By: reames
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D118604