[DSE] Lift post-dominance for objs not accessible in caller.

We can eliminate MemoryDefs of objects not accessible after the function
returns (e.g. alloca), if there are no reads between the MemoryDef and
any function exits. We can stop traversing paths that completely
overwrite the memory location of the MemoryDef.

This patch was split off D73763.

Reviewers: dmgreen, bryant, asbirlea, Tyker, efriedma, george.burgess.iv

Reviewed By: asbirlea, george.burgess.iv

Differential Revision: https://reviews.llvm.org/D77736