Fix SILCombine miscompile for discard on non-copyable types

[atrick]

Fixes rdar://113214179 (Raw layout types don't adhere to discard self)

// swift-frontend ./rawdeinit.swift -enable-experimental-feature RawLayout \ // -enable-builtin-module -emit-sil -o rawdeinit.sil

import Builtin

@_rawLayout(like: T)
public struct Cell<T>: ~Copyable {
  var ptr: UnsafeMutablePointer<T> {
    .init(Builtin.unprotectedAddressOfBorrow(self))
  }

  init(_ initialValue: T) {
    ptr.initialize(to: initialValue)
  }

  deinit {
    ptr.deinitialize(count: 1)
  }

  public consuming func get() -> T {
    let value = ptr.move()
    discard self // SILCombine inserts a destroy of self here
    return value
  }
}

[atrick]

@swift-ci test

[nate-chandler]

LGTM