[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

include/llvm/Transforms/Utils/SimplifyLibCalls.h

@@ -46,6 +46,7 @@ public:
   /// \brief Take the given call instruction and return a more
   /// optimal value to replace the instruction with or 0 if a more
   /// optimal form can't be found.
+  /// The call must not be an indirect call.
   Value *optimizeCall(CallInst *CI);
 
 private:
@@ -83,6 +84,7 @@ public:
   /// be equal to the instruction being optimized.  In this case all
   /// other instructions that use the given instruction were modified
   /// and the given instruction is dead.
+  /// The call must not be an indirect call.
   Value *optimizeCall(CallInst *CI);
 
   /// replaceAllUsesWith - This method is used when the library call