This is preparation for the next commit.

This saves a few blocks and matches the common `unpack!` paradigm.

…atthewjasper

match lowering: Clarify the main loop of the algorithm

Now that we expand or-patterns in a single place in the algorithm, we can move it (back) to the main part of the loop. This makes the call-graph of the main loop rather simple: `match_candidates` has three branches that each call back to `match_candidates`. The remaining tricky part is `finalize_or_candidate`.

I also factored out the whole "process a prefix of the candidates then process the rest" thing which I think helps legibility.

The first two commits are a fix for an indexing mistake I introduced in rust-lang#126553, already sumitted in rust-lang#127028 but feel free to merge this first.

r? `@matthewjasper`

as_simd: fix doc comment to be in line with align_to

In rust-lang#121201, the guarantees about `align_offset` and `align_to` were changed. This PR aims to correct the doc comment of `as_simd` to be in line with the new `align_to`.

Tagging rust-lang#86656 for good measure.

…errors

More suggestion for converting `Option<&Vec<T>>` to `Option<&[T]>`

Please review commit-by-commit.

…iser

compiletest: Better error message for bad `normalize-*` headers

Follow-up to rust-lang#126777.

Example of the new error message in context:
```text
---- [ui] tests/ui/rfcs/rfc-2632-const-trait-impl/effects/minicore.rs stdout ----
thread '[ui] tests/ui/rfcs/rfc-2632-const-trait-impl/effects/minicore.rs' panicked at src/tools/compiletest/src/header.rs:1001:13:
couldn't parse custom normalization rule: `normalize-stderr-test ".*note: .*\n\n" -> ""`
help: expected syntax is: `normalize-stderr-test: "REGEX" -> "REPLACEMENT"`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

…r=lqd

Mark `builtin_syntax` as internal

Tracking issue literally says:

> There will never be a general stabilization.

cc rust-lang#110680 `@est31`

… r=workingjubilee

Revert accidental comment deletion

This reverts an accidental comment deletion made in rust-lang#113128

See also rust-lang#113128 (comment)