Fix comparing comparisons

Sometimes the result of a comparison is not directly used in a jump,
but in arithmetic or further comparisons.  If those further things
do a vswap() with the VT_CMP as current top, and then generate
instructions for the new top, this most probably destroys the flags
(e.g. if it's a bitfield load like in the example).

vswap() must do the same like vsetc() and not allow VT_CMP vtops
to be moved down.

tccgen.c

@@ -452,6 +452,14 @@ ST_FUNC void vswap(void)
 {
     SValue tmp;
 
+    /* cannot let cpu flags if other instruction are generated. Also
+       avoid leaving VT_JMP anywhere except on the top of the stack
+       because it would complicate the code generator. */
+    if (vtop >= vstack) {
+        int v = vtop->r & VT_VALMASK;
+        if (v == VT_CMP || (v & ~1) == VT_JMP)
+            gv(RC_INT);
+    }
     tmp = vtop[0];
     vtop[0] = vtop[-1];
     vtop[-1] = tmp;