optimize reassignment immutable state
This is the "simple fix" when it comes to checking for reassignment. We just shoot for compatibility with the AST-based checker. Makes no attempt to solve #21232.
I opted for this simpler fix because I didn't want to think about complications [like the ones described here](https://github.com/rust-lang/rust/issues/21232#issuecomment-412219247).
Let's do some profiling measurements.
Fixes#53189
r? @pnkfelix
Implement Unsized Rvalues
This PR is the first step to implement RFC1909: unsized rvalues (#48055).
## Implemented
- `Sized` is removed for arguments and local bindings. (under `#![feature(unsized_locals)]`)
- Unsized locations are allowed in MIR
- Unsized places and operands are correctly translated at codegen
## Not implemented in this PR
- Additional `Sized` checks:
- tuple struct constructor (accidentally compiles now)
- closure arguments at closure generation (accidentally compiles now)
- upvars (ICEs now)
- Generating vtable for `fn method(self)` (ICEs now)
- VLAs: `[e; n]` where `n` isn't const
- Reduce unnecessary allocations
## Current status
- [x] Fix `__rust_probestack` (rust-lang-nursery/compiler-builtins#244)
- [x] Get the fix merged
- [x] `#![feature(unsized_locals)]`
- [x] Give it a tracking issue number
- [x] Lift sized checks in typeck and MIR-borrowck
- [ ] <del>Forbid `A(unsized-expr)`</del> will be another PR
- [x] Minimum working codegen
- [x] Add more examples and fill in unimplemented codegen paths
- [ ] <del>Loosen object-safety rules (will be another PR)</del>
- [ ] <del>Implement `Box<FnOnce>` (will be another PR)</del>
- [ ] <del>Reduce temporaries (will be another PR)</del>
