[AMDGPU] Request module used variables from LDS lowering as internal

I do not see any practical difference but technically
used.* variables are internal and a call to getGlobalVariable
misses true as a second argument. NFC as far as I can tell.

Differential Revision: https://reviews.llvm.org/D102884

llvm/lib/Target/AMDGPU/AMDGPULowerModuleLDSPass.cpp

@@ -52,7 +52,7 @@ class AMDGPULowerModuleLDS : public ModulePass {
 
   static void removeFromUsedList(Module &M, StringRef Name,
                                  SmallPtrSetImpl<Constant *> &ToRemove) {
-    GlobalVariable *GV = M.getGlobalVariable(Name);
+    GlobalVariable *GV = M.getNamedGlobal(Name);
     if (!GV || ToRemove.empty()) {
       return;
     }