Correct the recursive PHI node handling routines in a way that CANNOT induce

Correct the recursive PHI node handling routines in a way that CANNOT induce
infinite loops (using the new replaceSymbolicValuesWithConcrete method).

This patch reverts this patch:
http://mail.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20050131/023830.html

... which was an attempted fix for this problem.  Unfortunately, that patch
caused test/Regression/Transforms/IndVarsSimplify/exit_value_tests.llx to fail
and slightly castrated the entire analysis.  This patch fixes it right.

This patch is dedicated to jeffc, for making me deal with this.  :)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@20146 91177308-0d34-0410-b5e6-96231b3b80d8