[Attributor][Fix] Add alignment return attribute to HeapToStack

This patch changes the HeapToStack optimization to attach the return alignment
attribute information to the created alloca instruction. This would cause
problems when replacing the heap allocation with an alloca did not respect the
alignment of the original heap allocation, which would typically be aligned on
an 8 or 16 byte boundary. Malloc calls now contain alignment attributes,
so we can use that information here.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D115888