How to compress the size of a field in a structure?

[Hanke Zhang]

Hi, I'm attempting to compress the size of a field in a structure for
memory-friendly purposes. I created an IPA pass to achieve this, but I
ran into some issues as follows:

// original
struct Foo {
  long a1;
  int a2;
};

// modified
struct Foo_update {
  int a1;
  int a2;
};

For the example structure Foo, I use `TREE_TYPE (field) =
integer_type_node` to compress the type of a1 from `long` to `int`.

But I don't know how to update its corresponding SSA variables,
because the number of them is huge. Is there any way to do it quickly?

Thanks
Hanke Zhang