[LIR] Add support for structs and hand unrolled loops

[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;
  }
}

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258620 91177308-0d34-0410-b5e6-96231b3b80d8