IR. Change strip* family of functions to not look through aliases.

I noticed another instance of the issue where references to aliases were
being replaced with aliasees, this time in InstCombine. In the instance that
I saw it turned out to be only a QoI issue (a symbol ended up being missing
from the symbol table due to the last reference to the alias being removed,
preventing HWASAN from symbolizing a global reference), but it could easily
have manifested as incorrect behaviour.

Since this is the third such issue encountered (previously: D65118, D65314)
it seems to be time to address this common error/QoI issue once and for all
and make the strip* family of functions not look through aliases.

Includes a test for the specific issue that I saw, but no doubt there are
other similar bugs fixed here.

As with D65118 this has been tested to make sure that the optimization isn't
load bearing. I built Clang, Chromium for Linux, Android and Windows as well
as the test-suite and there were no size regressions.

Differential Revision: https://reviews.llvm.org/D66606

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369697 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/Module.cpp

@@ -604,7 +604,7 @@ GlobalVariable *llvm::collectUsedGlobalVariables(
 
   const ConstantArray *Init = cast<ConstantArray>(GV->getInitializer());
   for (Value *Op : Init->operands()) {
-    GlobalValue *G = cast<GlobalValue>(Op->stripPointerCastsNoFollowAliases());
+    GlobalValue *G = cast<GlobalValue>(Op->stripPointerCasts());
     Set.insert(G);
   }
   return GV;