[IR] Revert r2694 in BasicBlock::removePredecessor

[foad]

Summary

r2694 fixed a bug where removePredecessor could create IR with a use not
dominated by its def in a self loop. But this could only happen in an
unreachable loop, and since that time the rules have been relaxed so
that defs don't have to dominate uses in unreachable code, so the fix is
unnecessary. The regression test added in r2691 still stands.

[stephenneuendorffer]

Huzzah! Code simplification!

[lattner]

Thank you for cleaning this up!

llvm/lib/IR/BasicBlock.cpp

assert((hasNUsesOrMore(16)||// Reduce cost of this assertion for complex CFGs.
           find(pred_begin(this), pred_end(this), Pred) != pred_end(this)) &&
          "removePredecessor: BB is not a predecessor!");
 
   if (InstList.empty()) return;
   PHINode *APN = dyn_cast<PHINode>(&front());
   if (!APN) return;   // Quick exit.
 
   // If there are exactly two predecessors, then we want to nuke the PHI nodes
-  // altogether.  However, we cannot do this, if this in this case:
-  //
-  //  Loop:
-  //    %x = phi [X, Loop]
-  //    %x2 = add %x, 1         ;; This would become %x2 = add %x2, 1
-  //    br Loop                 ;; %x2 does not dominate all uses
-  //
-  // This is because the PHI node input is actually taken from the predecessor
-  // basic block.  The only case this can happen is with a self loop, so we
-  // check for this case explicitly now.
-  //
+  // altogether.
   unsigned max_idx = APN->getNumIncomingValues();
   assert(max_idx != 0 && "PHI Node in block with 0 predecessors!?!?!");
-  if (max_idx == 2) {
-    BasicBlock *Other = APN->getIncomingBlock(APN->getIncomingBlock(0) == Pred);
-
-    // Disable PHI elimination!
-    if (this == Other) max_idx = 3;
-  }
 
   // <= Two predecessors BEFORE I remove one?
   if (max_idx <= 2 && !KeepOneInputPHIs) {
     // Yup, loop through and nuke the PHI nodes
     while (PHINode *PN = dyn_cast<PHINode>(&front())) {
       // Remove the predecessor first.
       PN->removeIncomingValue(Pred, !KeepOneInputPHIs);