[asan] don't unpoison redzones on function exit in use-after-return mode.

Summary:
Before this change the instrumented code before Ret instructions looked like:
  <Unpoison Frame Redzones>
  if (Frame != OriginalFrame) // I.e. Frame is fake
     <Poison Complete Frame>

Now the instrumented code looks like:
  if (Frame != OriginalFrame) // I.e. Frame is fake
     <Poison Complete Frame>
  else
     <Unpoison Frame Redzones>

Reviewers: eugenis

Reviewed By: eugenis

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2458

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197907 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Transforms/Utils/BasicBlockUtils.h

@@ -205,6 +205,26 @@ TerminatorInst *SplitBlockAndInsertIfThen(Value *Cond, Instruction *SplitBefore,
                                           bool Unreachable,
                                           MDNode *BranchWeights = 0);
 
+
+/// SplitBlockAndInsertIfThenElse is similar to SplitBlockAndInsertIfThen,
+/// but also creates the ElseBlock.
+/// Before:
+///   Head
+///   SplitBefore
+///   Tail
+/// After:
+///   Head
+///   if (Cond)
+///     ThenBlock
+///   else
+///     ElseBlock
+///   SplitBefore
+///   Tail
+void SplitBlockAndInsertIfThenElse(Value *Cond, Instruction *SplitBefore,
+                                   TerminatorInst **ThenTerm,
+                                   TerminatorInst **ElseTerm,
+                                   MDNode *BranchWeights = 0);
+
 ///
 /// GetIfCondition - Check whether BB is the merge point of a if-region.
 /// If so, return the boolean condition that determines which entry into