Implement more aggressive support for analyzing string length. This

Implement more aggressive support for analyzing string length.  This
generalizes the previous code to handle the case when the string is not
an immediate to the strlen call (for example, crazy stuff like
strlen(c ? "foo" : "bart"+1) -> 3).  This implements
gcc.c-torture/execute/builtins/strlen-2.c.  I will generalize other
cases in simplifylibcalls to use the same routine later.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@50408 91177308-0d34-0410-b5e6-96231b3b80d8