[LIR] Add support for structs and hand unrolled loops

Now LIR can turn following codes into memset:

typedef struct foo {

int a;
int b;

} foo_t;

void bar(foo_t *f, unsigned n) {

for (unsigned i = 0; i < n; ++i) {
  f[i].a = 0;
  f[i].b = 0;
}

}

void test(foo_t *f, unsigned n) {

for (unsigned i = 0; i < n; i += 2) {
  f[i] = 0;
  f[i+1] = 0;
}

}