[SimplifyLibCalls] Don't try to simplify indirect calls.

[SimplifyLibCalls] Don't try to simplify indirect calls.

It turns out, all callsites of the simplifier are guarded by a check for
CallInst::getCalledFunction (i.e., to make sure the callee is direct).

This check wasn't done when trying to further optimize a simplified fortified
libcall, introduced by a refactoring in r225640.

Fix that, add a testcase, and document the requirement.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225895 91177308-0d34-0410-b5e6-96231b3b80d8