GVN invalidates memdep

[lattner]

Bugzilla Link	3174
Version	1.0
OS	All

Extended Description

The GVN pass is invalidating memdep, which causes it to be recomputed for memcpy optimizer. We also run DSE substantially later, which means that extra passes have to get recomputed:

    Memory Dependence Analysis
    Global Value Numbering

•  Memory Dependence Analysis
   MemCpy Optimization
   Sparse Conditional Constant Propagation
   Combine redundant instructions
   Break critical edges in CFG
   Conditional Propagation
  

•  Dominator Tree Construction
   Memory Dependence Analysis
   Dead Store Elimination
  

GVN would have to inform memdep about blocks it removes if it were to declare that it preserved it though (MergeBlockIntoPredecessor) and we'd have to teach memdep how to handle this.