Use Symbol for inline asm register class names

This takes care of one "FIXME":
// FIXME: use direct symbol comparison for register class names

Instead of using string literals, this uses Symbol for register
class names.

compiler/rustc_span/src/symbol.rs

@@ -460,6 +460,9 @@ symbols! {
         document_private_items,
         dotdot_in_tuple_patterns,
         dotdoteq_in_patterns,
+        dreg,
+        dreg_low16,
+        dreg_low8,
         drop,
         drop_in_place,
         drop_types_in_const,
@@ -544,6 +547,7 @@ symbols! {
         format_args_capture,
         format_args_nl,
         freeze,
+        freg,
         frem_fast,
         from,
         from_desugaring,
@@ -627,6 +631,7 @@ symbols! {
         iter,
         keyword,
         kind,
+        kreg,
         label,
         label_break_value,
         lang,
@@ -652,6 +657,7 @@ symbols! {
         lint_reasons,
         literal,
         llvm_asm,
+        local,
         local_inner_macros,
         log10f32,
         log10f64,
@@ -854,6 +860,9 @@ symbols! {
         pub_restricted,
         pure,
         pushpop_unsafe,
+        qreg,
+        qreg_low4,
+        qreg_low8,
         quad_precision_float,
         question_mark,
         quote,
@@ -875,6 +884,13 @@ symbols! {
         reexport_test_harness_main,
         reference,
         reflect,
+        reg,
+        reg16,
+        reg32,
+        reg64,
+        reg_abcd,
+        reg_byte,
+        reg_thumb,
         register_attr,
         register_tool,
         relaxed_adts,
@@ -1060,6 +1076,8 @@ symbols! {
         spotlight,
         sqrtf32,
         sqrtf64,
+        sreg,
+        sreg_low16,
         sse4a_target_feature,
         stable,
         staged_api,
@@ -1215,6 +1233,8 @@ symbols! {
         volatile_load,
         volatile_set_memory,
         volatile_store,
+        vreg,
+        vreg_low16,
         warn,
         wasm_import_module,
         wasm_target_feature,
@@ -1226,6 +1246,9 @@ symbols! {
         wrapping_mul,
         wrapping_sub,
         write_bytes,
+        xmm_reg,
+        ymm_reg,
+        zmm_reg,
     }
 }