fix crash in tail call elimination (PR22704)

[spatel]

Summary

This is a trivial fix (I hope), but I'm sending this patch for review to draw the attention of people more familiar with TailRecursionElimination.

I suspect there are other similar dyn_cast bugs lurking in this pass.

[spatel]

Fixed better by Benjamin Kramer:
http://llvm.org/viewvc/llvm-project?rev=230856&view=rev

lib/Transforms/Scalar/TailRecursionElimination.cpp

 /// Check to see if the function containing the specified tail call consistently
 /// returns the same runtime-constant value at all exit points except for
 /// IgnoreRI. If so, return the returned value.
 static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) {
   Function *F = CI->getParent()->getParent();
   Value *ReturnedValue = nullptr;
 
   for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) {
-    ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator());
+    ReturnInst *RI = dyn_cast_or_null<ReturnInst>(BBI->getTerminator());
     if (RI == nullptr || RI == IgnoreRI) continue;
 
     // We can only perform this transformation if the value returned is
     // evaluatable at the start of the initial invocation of the function,
     // instead of at the end of the evaluation.
     //
     Value *RetOp = RI->getOperand(0);
     if (!isDynamicConstant(RetOp, CI, RI))

test/Transforms/TailCallElim/inf-recursion.ll

 ; CHECK-NOT: call
 ; CHECK: }
 define float @fabsf(float %f) {
         %t= call float @fabsf(float 2.0)
         ret float %t
 }
 
 declare x86_fp80 @fabsl(x86_fp80 %f)
+
+
+; Don't crash while transforming a function with infinite recursion.
+
+define i32 @PR22704(i1 %bool) {
+entry:
+  br i1 %bool, label %t, label %f
+
+t:
+  %call1 = call i32 @PR22704(i1 1)
+  br label %return
+
+f:
+  %call = call i32 @PR22704(i1 1)
+  br label %return
+
+return:
+  ret i32 0
+
+; CHECK-LABEL: @PR22704(
+; CHECK:       %bool.tr = phi i1 [ %bool, %entry ], [ true, %t ], [ true, %f ]
+; CHECK:       br i1 %bool.tr, label %t, label %f
+}
+