Fix a few things with -Winfinite-recursion. NFC

Now that -Winfinite-recursion no longer uses recursive calls to before path
analysis, several bits of the code can be improved.  The main changes:

1) Early return when finding a path to the exit block without a recursive call
2) Moving the states vector into checkForRecursiveFunctionCall instead of
   passing it in by reference
3) Change checkForRecursiveFunctionCall to return a bool when the warning
   should be emitted.
4) Use the State vector instead of storing it in the Stack vector.

llvm-svn: 245666