[Attributor][FIX] Heap2Stack needs to use the alloca AS

When we move an allocation from the heap to the stack we need to
allocate it in the alloca AS and then cast the result. This also
prevents us from inserting the alloca after the allocation call but
rather right before.

Fixes https://github.com/llvm/llvm-project/issues/53858

(cherry picked from commit 8ad39fba)