remove struct_gep, use manual layout calculations for va_arg

compiler/rustc_codegen_llvm/src/builder.rs

@@ -778,11 +778,6 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         }
     }
 
-    fn struct_gep(&mut self, ty: &'ll Type, ptr: &'ll Value, idx: u64) -> &'ll Value {
-        assert_eq!(idx as c_uint as u64, idx);
-        unsafe { llvm::LLVMBuildStructGEP2(self.llbuilder, ty, ptr, idx as c_uint, UNNAMED) }
-    }
-
     /* Casts */
     fn trunc(&mut self, val: &'ll Value, dest_ty: &'ll Type) -> &'ll Value {
         unsafe { llvm::LLVMBuildTrunc(self.llbuilder, val, dest_ty, UNNAMED) }