[PATCH] D18065: Fix bugs in the MemorySSA walker

[George Burgess IV via llvm-commits]

george.burgess.iv created this revision.
george.burgess.iv added a reviewer: dberlin.
george.burgess.iv added a subscriber: llvm-commits.

Test case:

```
define void @foo(i1 %b, i8* %ext) {
  %a = alloca i8, align 1
  %sink = alloca i8, align 1
  ; 1 = MemoryDef(liveOnEntry) <<<<<<<<
  store i8 1, i8* %a, align 1
  br i1 %b, label %if.b, label %end

if.b:
  ; 2 = MemoryDef(1)
  store i8 1, i8* %sink
  br label %end

end:
  ; 3 = MemoryPhi({if.b,2},{%0,1})
  ; MemoryUse(liveOnEntry)  <<<<<<<<<<<<<
  load i8, i8* %a, align 1
  ret void
}
```

...This happens because because of how we recursively walk phis, and how we cache the results afterward. Both of these problems are (hopefully) fixed by this patch.

I plan to move the code around in the loop a bit (because `break;` as the last statement in a `for` is kind of dumb), but I’ve separated that from this patch because the refactor adds a fair amount of noise, and should be NFC.

http://reviews.llvm.org/D18065

Files:
  include/llvm/Transforms/Utils/MemorySSA.h
  lib/Transforms/Utils/MemorySSA.cpp
  test/Transforms/Util/MemorySSA/cyclicphi.ll
  test/Transforms/Util/MemorySSA/phi-translation.ll

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