[llvm] [IsolatePath] Add a pass to isolate paths with undefined behavior (PR #146791)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> foo:
  %phi.val = phi ptr [ %arrayidx.i, %pred1 ], [ null, %pred2 ]
  %load.val = load i32, ptr %phi.val, align 4
   ...

This pass should run before SimplifyCFGPass. Otherwise, `%pred2 -> %foo` will be removed in `removeUndefIntroducingPredecessor`: https://github.com/llvm/llvm-project/blob/2dba752f0575306d8178e9bb4869d06528ab452c/llvm/lib/Transforms/Utils/SimplifyCFG.cpp#L8156-L8168


https://github.com/llvm/llvm-project/pull/146791