tccgen: use lvalue as result from bitfield assignment

test case:

    #include <stdio.h>
    int main(int argc, char **argv)
    {
        struct _s { unsigned a:9, b:5, c:7; } _s, *s = &_s;
        int n = 250;
        s->a = s->b = s->c = n + 4;
        printf("--> %d / %d / %d\n", s->a, s->b, s->c);
        return 0;
    }

before:
--> 254 / 30 / 126
now:
--> 30 / 30 / 126

tccgen.c

@@ -2520,16 +2520,15 @@ ST_FUNC void vstore(void)
         /* leave source on stack */
     } else if (ft & VT_BITFIELD) {
         /* bitfield store handling */
+
+        /* save lvalue as expression result (example: s.b = s.a = n;) */
+        vdup(), vtop[-1] = vtop[-2];
+
         bit_pos = (ft >> VT_STRUCT_SHIFT) & 0x3f;
         bit_size = (ft >> (VT_STRUCT_SHIFT + 6)) & 0x3f;
         /* remove bit field info to avoid loops */
         vtop[-1].type.t = ft & ~(VT_BITFIELD | (-1 << VT_STRUCT_SHIFT));
 
-        /* duplicate source into other register */
-        gv_dup();
-        vswap();
-        vrott(3);
-
         if((ft & VT_BTYPE) == VT_BOOL) {
             gen_cast(&vtop[-1].type);
             vtop[-1].type.t = (vtop[-1].type.t & ~VT_BTYPE) | (VT_BYTE | VT_UNSIGNED);
@@ -2561,8 +2560,7 @@ ST_FUNC void vstore(void)
         gen_op('|');
         /* store result */
         vstore();
-
-        /* pop off shifted source from "duplicate source..." above */
+        /* ... and discard */
         vpop();
 
     } else {