Revert "[IfConversion] Keep the CFG updated incrementally in IfConvertTriangle"

The reverted change introdued assertions ala:

"MachineBasicBlock::succ_iterator
llvm::MachineBasicBlock::removeSuccessor(succ_iterator, bool): Assertion
`I != Successors.end() && "Not a current successor!"'

Mikael, the original committer, wrote me that he is working on a fix, but that
it likely will take some time to get this resolved. As this bug is one of the
last two issues that keep the AOSP buildbot from turning green, I revert the
original commit r302876.

I am looking forward to see this recommitted after the assertion has been
resolved.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304128 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/IfConversion.cpp

@@ -1588,32 +1588,22 @@ bool IfConverter::IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind) {
     BBCvt = MBPI->getEdgeProbability(BBI.BB, &CvtMBB);
   }
 
-  // To be able to insert code freely at the end of BBI we sometimes remove
-  // the branch from BBI to NextMBB temporarily. Remember if this happened.
-  bool RemovedBranchToNextMBB = false;
   if (CvtMBB.pred_size() > 1) {
     BBI.NonPredSize -= TII->removeBranch(*BBI.BB);
     // Copy instructions in the true block, predicate them, and add them to
     // the entry block.
     CopyAndPredicateBlock(BBI, *CvtBBI, Cond, true);
 
-    // Keep the CFG updated.
+    // RemoveExtraEdges won't work if the block has an unanalyzable branch, so
+    // explicitly remove CvtBBI as a successor.
     BBI.BB->removeSuccessor(&CvtMBB, true);
   } else {
     // Predicate the 'true' block after removing its branch.
     CvtBBI->NonPredSize -= TII->removeBranch(CvtMBB);
     PredicateBlock(*CvtBBI, CvtMBB.end(), Cond);
 
-    // Remove the branch from the entry of the triangle to NextBB to be able to
-    // do the merge below. Keep the CFG updated, but remember we removed the
-    // branch since we do want to execute NextMBB, either by introducing a
-    // branch to it again, or merging it into the entry block.
-    // How it's handled is decided further down.
-    BBI.NonPredSize -= TII->removeBranch(*BBI.BB);
-    BBI.BB->removeSuccessor(&NextMBB, true);
-    RemovedBranchToNextMBB = true;
-
     // Now merge the entry of the triangle with the true block.
+    BBI.NonPredSize -= TII->removeBranch(*BBI.BB);
     MergeBlocks(BBI, *CvtBBI, false);
   }
 
@@ -1651,19 +1641,12 @@ bool IfConverter::IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind) {
     // block. By not merging them, we make it possible to iteratively
     // ifcvt the blocks.
     if (!HasEarlyExit &&
-        // We might have removed BBI from NextMBB's predecessor list above but
-        // we want it to be there, so consider that too.
-        (NextMBB.pred_size() == (RemovedBranchToNextMBB ? 0 : 1)) &&
-        !NextBBI->HasFallThrough &&
+        NextMBB.pred_size() == 1 && !NextBBI->HasFallThrough &&
         !NextMBB.hasAddressTaken()) {
-      // We will merge NextBBI into BBI, and thus remove the current
-      // fallthrough from BBI into CvtBBI.
-      BBI.BB->removeSuccessor(&CvtMBB, true);
       MergeBlocks(BBI, *NextBBI);
       FalseBBDead = true;
     } else {
       InsertUncondBranch(*BBI.BB, NextMBB, TII);
-      BBI.BB->addSuccessor(&NextMBB);
       BBI.HasFallThrough = false;
     }
     // Mixed predicated and unpredicated code. This cannot be iteratively
@@ -1671,6 +1654,8 @@ bool IfConverter::IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind) {
     IterIfcvt = false;
   }
 
+  RemoveExtraEdges(BBI);
+
   // Update block info. BB can be iteratively if-converted.
   if (!IterIfcvt)
     BBI.IsDone = true;

test/CodeGen/MIR/ARM/PR32721_ifcvt_triangle_unanalyzable.mir

@@ -1,24 +0,0 @@
-# RUN: llc -mtriple=arm-apple-ios -run-pass=if-converter %s -o - | FileCheck %s
----
-name:            foo
-body:             |
-  bb.0:
-    B %bb.2
-
-  bb.1:
-    BX_RET 14, 0
-
-  bb.2:
-    Bcc %bb.1, 1, %cpsr
-
-  bb.3:
-    B %bb.1
-
-...
-
-# We should get a single block containing the BX_RET, with no successors at all
-
-# CHECK:      body:
-# CHECK-NEXT:   bb.0:
-# CHECK-NEXT:     BX_RET
-