Change how PHINodes store their operands.

Change PHINodes to store simple pointers to their incoming basic blocks,
instead of full-blown Uses.

Note that this loses an optimization in SplitCriticalEdge(), because we
can no longer walk the use list of a BasicBlock to find phi nodes. See
the comment I removed starting "However, the foreach loop is slow for
blocks with lots of predecessors".

Extend replaceAllUsesWith() on a BasicBlock to also update any phi
nodes in the block's successors. This mimics what would have happened
when PHINodes were proper Users of their incoming blocks. (Note that
this only works if OldBB->replaceAllUsesWith(NewBB) is called when
OldBB still has a terminator instruction, so it still has some
successors.)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133435 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/CloneFunction.cpp

@@ -572,12 +572,12 @@ void llvm::CloneAndPruneFunctionInto(Function *NewFunc, const Function *OldFunc,
     // removed, so we just need to splice the blocks.
     BI->eraseFromParent();
     
-    // Move all the instructions in the succ to the pred.
-    I->getInstList().splice(I->end(), Dest->getInstList());
-    
     // Make all PHI nodes that referred to Dest now refer to I as their source.
     Dest->replaceAllUsesWith(I);
 
+    // Move all the instructions in the succ to the pred.
+    I->getInstList().splice(I->end(), Dest->getInstList());
+    
     // Remove the dest block.
     Dest->eraseFromParent();