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