[If Converter] Convert recursion to iteration.

This commit makes changes to IfConverter::AnalyzeBlock to use iteration instead
of recursion. Previously, this function would get called recursively a large
number of times and eventually segfault when a function with the following CFG
was compiled:

BB0:
 if (condition0)
  goto BB1
 goto BB2
BB1:
 goto BB2
BB2:
 if (condition1)
  goto BB3
 goto BB4
BB3:
...
(repeat until BB7488)

rdar://problem/21386145

Differential Revision: http://reviews.llvm.org/D10587

llvm-svn: 240589