[DA] Refactor with a better API

Refactor from iteratively using BitCastInst::getOperand()
to using stripPointerCasts() instead. This is an improvement
since now we are able to analyze more cases, please refer
to test cases added in this patch.

Reviewed By: Meinersbur, #loopoptwg

Differential Revision: https://reviews.llvm.org/D123559