Reduces the amount of wasted processor cycles

Co-authored-by: kennytm <kennytm@gmail.com>

Change cg_ssa's get_param to borrow the builder mutably

This is a small change to make `get_param` more flexible for codegens that may need to modify things when retrieving function parameters.

This will currently only be used by [rustc_codegen_nvvm](https://github.com/Rust-GPU/Rust-CUDA) (my own project), but may be useful to more codegens in the future.

This is needed because cg_nvvm needs to remap certain types to libnvvm-friendly types, such as `i128` -> `<2 x i64>`. Because cg_ssa does not give mutable access to the builder, i resorted to using a mutex:
```rs
    fn get_param(&self, index: usize) -> Self::Value {
        let val = llvm::get_param(self.llfn(), index as c_uint);
        trace!("Get param `{:?}`", val);
        unsafe {
            let llfnty = LLVMRustGetFunctionType(self.llfn());
            let map = self.remapped_integer_args.borrow();
            if let Some((_, key)) = map.get(llfnty) {
                if let Some((_, new_ty)) = key.iter().find(|t| t.0 == index) {
                    trace!("Casting irregular param {:?} to {:?}", val, new_ty);
                    return transmute_llval(
                        *self.llbuilder.lock().unwrap(),
                        &self.cx,
                        val,
                        *new_ty,
                    );
                }
            }
            val
        }
    }
```
However, i predict this is pretty bad for performance, considering how much builders are called during codegen, so i would greatly appreciate having a more flexible API for this.

If the thread does not get the lock in the short term, yield the CPU

Reduces on [RustyHermit](https://github.com/hermitcore/rusty-hermit) the amount of wasted processor cycles

Fix ICE due to out-of-bounds statement index when reporting borrowck error

Replace an `[index]` with a `.get` when `statement_index` points to a basic-block terminator (and is therefore out-of-bounds in the statements list).

Fixes rust-lang#91206
Cc ``@camsteffen``
r? ``@oli-obk``

…r, r=jsha

Fix invalid scrollbar display on source code page

Fixes bug introduced in rust-lang#90983:

![Screenshot from 2021-11-25 17-01-08](https://user-images.githubusercontent.com/3050060/143473753-c2e7c43c-ce3f-474d-9d2a-922e63189c51.png)
![Screenshot from 2021-11-25 17-07-08](https://user-images.githubusercontent.com/3050060/143473757-eecaaf2b-f4f0-49e0-a159-ab485e3f7122.png)

To fix it, I simply unset the `overflow-y` on the source code page so it's not displayed anymore.

r? ``@jsha``