[SILGen] Simpler impl of multiple matching patterns in case w/ variable bindings

[gregomni]

Backed off of the implementation with a shared body block, which got messy really fast with multiple patterns containing address-only values to pass in, as it led to separated stack alloc/cleanup requirements and passing stack alloc’d values as block args, aliasing of addresses for stack cleanup verifying, etc. A mess.

This now does the conservative thing of repeating the body in each matching pattern block.

This can probably be profitably revisited in the future if @rjmccall 's recently suggested changes to make all SIL types into scalars gets implemented, which would make doing the shared body block style much easier (at least as far as SILGen is concerned).

Resolves SR-2555.

[gregomni]

@swift-ci Please smoke test and merge

[rjmccall]

That's unfortunate. Re-emitting the whole case block is potentially a really big hit.

I guess it'll do for now.

[jckarter]

We can't do this, because we're not set up to re-emit AST nodes in general, nor do we want to. This breaks if you have any closures or var decls in the case body, e.g.:

func foo(x: Foo) {
  switch x {
  case .a(let x), .b(let x):
    let y = x
  }
}

which is breaking some of our users at Apple. I'm going to have to revert this.

[jrose-apple]

What's the plan for SR-2555, then? The old code apparently wasn't correct either.

[jckarter]

Hopefully @gregomni is still working on this. A few weeks after this PR, he posted to swift-dev about fixing some of the issues he alluded to in the PR description, extending IRGen and the SIL verifier to understand phi'ed stack slots.