[analyzer] Clean up modeling of strcmp, including cases where a string literal has...

[analyzer] Clean up modeling of strcmp, including cases where a string literal has an embedded null character, and where both arguments are the same buffer. Also use nested ifs rather than early returns; in this case early returns will lose any assumptions we've made earlier in the function.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@133154 91177308-0d34-0410-b5e6-96231b3b80d8