[NFCI][SCEV] getPointerBase(): de-recursify

Summary:
This is boringly straight-forward, each iteration we see if
V is some expression that we can look into, and if it has
a single pointer operand, then set V to that operand
and repeat.

Reviewers: efriedma, mkazantsev, reames, nikic

Reviewed By: nikic

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82632