[SafeStack] Don't create SCEV min between pointer and integer (PR54784)

Rather than rewriting the alloca pointer to zero, use
removePointerBase() to drop the base pointer. This will simply bail
if the base pointer is not the alloca. We could try doing something
more fancy here (like dropping the sources not based on the alloca
on the premise that they aren't SafeStack-relevant), but I don't
think that's worthwhile.

Fixes https://github.com/llvm/llvm-project/issues/54784.

Differential Revision: https://reviews.llvm.org/D123309