[NFC][StackSafety] Test that StackLifetime looks through stripPointerCasts

StackLifetime class collects lifetime marker of an `alloca` by collect
the user of `BitCast` who is the user of the `alloca`. However, either
the `alloca` itself could be used with the lifetime marker or the `BitCast`
of the `alloca` could be transformed to other instructions. (e.g.,
it may be transformed to all zero reps in `InstCombine` pass).
This patch tries to fix this process in `collectMarkers` functions.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D85399