[AA][Intrinsics] Add separate_storage assumptions.

This operand bundle on an assume informs alias analysis that the
arguments point to regions of memory that were allocated separately
(i.e. different heap allocations, different allocas, or different
globals).

As a safety measure, we leave the analysis flag-disabled by default.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D136514