Fix a PHI bug in the BlockDisambiguate pass.

If we have a high-level block, BBX, that loops to itself, then the
BlockDisambiguate pass will insert a disambiguation block, BBY, for it.

But if there are PHI nodes at the top of BBX with incoming values from
the bottom of BBX, then we need to update the incoming block from BBX to
BBY.

We were failing to do this and it was making the compiler crash when I
amended a test to use YkLocations.

Author: vext01

llvm/lib/Transforms/Yk/BlockDisambiguate.cpp

@@ -204,6 +204,7 @@ class YkBlockDisambiguate : public ModulePass {
           if (SuccBB == &BB) {
             BasicBlock *DBB = makeDisambiguationBB(Context, &BB, NewBBs);
             BI->setSuccessor(SuccIdx, DBB);
+            BB.replacePhiUsesWith(&BB, DBB);
           }
         }
       } else if (isa<SwitchInst>(TI)) {
@@ -214,6 +215,7 @@ class YkBlockDisambiguate : public ModulePass {
           if (SuccBB == &BB) {
             BasicBlock *DBB = makeDisambiguationBB(Context, &BB, NewBBs);
             SI->setSuccessor(SuccIdx, DBB);
+            BB.replacePhiUsesWith(&BB, DBB);
           }
         }
       }