[AMDGPU] Fix a warning

This patch fixes:

  llvm/lib/Target/AMDGPU/AMDGPULowerModuleLDSPass.cpp:1031:17: error:
  unused variable 'F' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/Target/AMDGPU/AMDGPULowerModuleLDSPass.cpp

@@ -1028,8 +1028,7 @@ class AMDGPULowerModuleLDS {
     }
     // Also erase those special LDS variables from indirect_access.
     for (auto &K : LDSUsesInfo.indirect_access) {
-      Function *F = K.first;
-      assert(isKernelLDS(F));
+      assert(isKernelLDS(K.first));
       for (GlobalVariable *GV : K.second) {
         if (isNamedBarrier(*GV))
           K.second.erase(GV);