Make sure global constants are recognized as such in our own pass

The metadata on the load may be striped by LLVM when moving code around

src/cgutils.cpp

@@ -234,6 +234,11 @@ static Value *julia_pgv(jl_codectx_t &ctx, const char *cname, void *addr)
         gv = new GlobalVariable(*M, T_pjlvalue,
                                 false, GlobalVariable::PrivateLinkage,
                                 NULL, localname);
+    // LLVM passes sometimes strip metadata when moving load around
+    // since the load at the new location satisfy the same condition as the origional one.
+    // Mark the global as constant to LLVM code using our own metadata
+    // which is much less likely to be striped.
+    gv->setMetadata("julia.constgv", MDNode::get(gv->getContext(), None));
     assert(localname == gv->getName());
     assert(!gv->hasInitializer());
     return gv;

src/llvm-late-gc-lowering.cpp

@@ -1124,11 +1124,12 @@ static bool isLoadFromConstGV(LoadInst *LI)
 {
     // We only emit single slot GV in codegen
     // but LLVM global merging can change the pointer operands to GEPs/bitcasts
-    if (!isa<GlobalVariable>(LI->getPointerOperand()->stripInBoundsOffsets()))
-        return false;
-    MDNode *TBAA = LI->getMetadata(LLVMContext::MD_tbaa);
-    if (isTBAA(TBAA, {"jtbaa_const"}))
-        return true;
+    if (auto gv = dyn_cast<GlobalVariable>(LI->getPointerOperand()->stripInBoundsOffsets())) {
+        MDNode *TBAA = LI->getMetadata(LLVMContext::MD_tbaa);
+        if (isTBAA(TBAA, {"jtbaa_const"}) || gv->getMetadata("julia.constgv")) {
+            return true;
+        }
+    }
     return false;
 }