[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