Fall back to the selection dag isel to select tail calls.

This shouldn't affect codegen for -O0 compiles as tail call markers are not
emitted in unoptimized compiles.  Testing with the external/internal nightly
test suite reveals no change in compile time performance.  Testing with -O1,
-O2 and -O3 with fast-isel enabled did not cause any compile-time or
execution-time failures.  All tests were performed on my x86 machine.
I'll monitor our arm testers to ensure no regressions occur there.

In an upcoming clang patch I will be marking the objc_autoreleaseReturnValue
and objc_retainAutoreleaseReturnValue as tail calls unconditionally.  While
it's theoretically true that this is just an optimization, it's an
optimization that we very much want to happen even at -O0, or else ARC
applications become substantially harder to debug.

Part of rdar://12553082

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169796 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/FastISel.h

@@ -131,6 +131,10 @@ public:
   /// into the current block.
   void recomputeInsertPt();
 
+  /// removeDeadCode - Remove all dead instructions between the I and E.
+  void removeDeadCode(MachineBasicBlock::iterator I,
+                      MachineBasicBlock::iterator E);
+
   struct SavePoint {
     MachineBasicBlock::iterator InsertPt;
     DebugLoc DL;
@@ -395,10 +399,6 @@ private:
 
   /// hasTrivialKill - Test whether the given value has exactly one use.
   bool hasTrivialKill(const Value *V) const;
-
-  /// removeDeadCode - Remove all dead instructions between the I and E.
-  void removeDeadCode(MachineBasicBlock::iterator I,
-                      MachineBasicBlock::iterator E);
 };
 
 }