Collect constants within global_asm! in collector

This is currently a no-op, but will be useful when const in `global_asm!`
can be pointers.

Author: nbdd0121

compiler/rustc_monomorphize/src/collector.rs

@@ -486,10 +486,19 @@ fn collect_items_rec<'tcx>(
             if let hir::ItemKind::GlobalAsm { asm, .. } = item.kind {
                 for (op, op_sp) in asm.operands {
                     match *op {
-                        hir::InlineAsmOperand::Const { .. } => {
-                            // Only constants which resolve to a plain integer
-                            // are supported. Therefore the value should not
-                            // depend on any other items.
+                        hir::InlineAsmOperand::Const { anon_const } => {
+                            match tcx.const_eval_poly(anon_const.def_id.to_def_id()) {
+                                Ok(val) => {
+                                    collect_const_value(tcx, val, &mut used_items);
+                                }
+                                Err(ErrorHandled::TooGeneric(..)) => {
+                                    span_bug!(*op_sp, "asm const cannot be resolved; too generic")
+                                }
+                                Err(err @ ErrorHandled::Reported(..)) => {
+                                    err.emit_note(tcx);
+                                    continue;
+                                }
+                            }
                         }
                         hir::InlineAsmOperand::SymFn { expr } => {
                             let fn_ty = tcx.typeck(item_id.owner_id).expr_ty(expr);