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.

llvm-svn: 50408