i386-gen: fix USE_EBX

Restore ebx from *ebp because alloca might change esp.

Also disable USE_EBX for upcoming release.

Actually the benefit is less than one would expect, it
appears that tcc can't do much with more than 3 registers
except with extensive use of long longs where the disassembly
looks much prettier (and shorter also).

Also: tccgen/expr_cond() : fix wrong gv/save_regs order

tccgen.c

@@ -5164,8 +5164,8 @@ static void expr_cond(void)
 #endif
             } else
                 rc = RC_INT;
-            save_regs(1);
             gv(rc);
+            save_regs(1);
             if (g)
                 gv_dup();
             tt = gvtst(1, 0);