Let's distinguish between the original pinned locals and the PinnedRegion locals.
The format need declarations if any are left over after transformations; the latter don't.
The previously emitted `ref *(TTo*)(&source)` only compiles when `source` is a local variable; otherwise C# complains about the memory not being pinned.
Note that we special-case local variables to keep the previous behavior around; this avoids pulling in `System.Runtime.CompilerServices.Unsafe.dll` when it's unnecessary.
Not every pinned region has a clean `P = null` assignment to mark its end.
If a second pinned region starts with the same variable `P`, consider that to mark the end of the previous pinned region for that variable.
Also, fix a bunch of special cases with empty pinned regions.
Unreachable code is not part of the dominator tree, which most of our transforms are based on.
In particular, dominance-based loop detection runs into the problem where unreachable code might have jumps into two independent loops. In that case, it's impossible to place the unreachable code in a way that avoids assertions / generating invalid C#.
We establish the invariant that all blocks in a BlockContainer must be statically reachable from the entry point (-> every block is part of the dominator tree). This means transforms no longer have to deal with special cases for unreachable code.
The "Remove dead and side effect free code" option still has an effect on dead stores, but unreachable code is now always removed (previously this also was dependent on this option).
This works if all addresses are immediately used in calls (as common with method calls on value-type,
which take 'this' by-reference); as long as the call doesn't return the reference again.
Closes#1136.