Improve IR support for structs

[dubiousconst282]

Structs are currently almost always keept in exposed local variable slots, making it difficult for transforms to check for legality since there's almost no data-flow info.

TODOs:

• Consider making Variables always "semi-exposed", so that they always provide a symbolic byref/pointer.
  • Removes the need for Var Load/Store/Addr instructions
  • Problem: would require the instantiation of a ByrefType for every type instance in a local var.
• Implement some kind of analysis like Memory SSA
  • Maintaining a parallel IR may be a bit inconvenient given the current infra. We may want instead to consider some hybrid with scalar SSA and the exposed flag to match heap objects, so that we can make assumptions like "Span<T> instances being always read-only".
    A problem with this hybrid is that we'd still need some way to take the SSA address when calling instance methods/field accesses, and if we're not careful this could bring more issues than it'd solve (consider inlining something like void M() { this = default; }).