tcg: Work around clang bug wrt enum ranges

A subsequent patch patch will change the type of REG from int
to enum TCGReg, which provokes the following bug in clang:

  https://llvm.org/bugs/show_bug.cgi?id=16154

Signed-off-by: Richard Henderson <rth@twiddle.net>

tcg/tcg.c

@@ -2059,9 +2059,9 @@ static void tcg_reg_alloc_op(TCGContext *s,
     } else {
         if (def->flags & TCG_OPF_CALL_CLOBBER) {
             /* XXX: permit generic clobber register list ? */ 
-            for(reg = 0; reg < TCG_TARGET_NB_REGS; reg++) {
-                if (tcg_regset_test_reg(tcg_target_call_clobber_regs, reg)) {
-                    tcg_reg_free(s, reg);
+            for (i = 0; i < TCG_TARGET_NB_REGS; i++) {
+                if (tcg_regset_test_reg(tcg_target_call_clobber_regs, i)) {
+                    tcg_reg_free(s, i);
                 }
             }
         }
@@ -2227,9 +2227,9 @@ static void tcg_reg_alloc_call(TCGContext *s, int nb_oargs, int nb_iargs,
     }
 
     /* clobber call registers */
-    for(reg = 0; reg < TCG_TARGET_NB_REGS; reg++) {
-        if (tcg_regset_test_reg(tcg_target_call_clobber_regs, reg)) {
-            tcg_reg_free(s, reg);
+    for (i = 0; i < TCG_TARGET_NB_REGS; i++) {
+        if (tcg_regset_test_reg(tcg_target_call_clobber_regs, i)) {
+            tcg_reg_free(s, i);
         }
     }