Auto merge of #83458 - saethlin:improve-vec-benches, r=dtolnay

Auto merge of #83458 - saethlin:improve-vec-benches, r=dtolnay

Clean up Vec's benchmarks

The Vec benchmarks need a lot of love. I sort of noticed this in https://github.com/rust-lang/rust/pull/83357 but the overall situation is much less awesome than I thought at the time. The first commit just removes a lot of asserts and does a touch of other cleanup.

A number of these benchmarks are poorly-named. For example, `bench_map_fast` is not in fact fast, `bench_rev_1` and `bench_rev_2` are vague, `bench_in_place_zip_iter_mut` doesn't call `zip`, `bench_in_place*` don't do anything in-place... Should I fix these, or is there tooling that depend on the names not changing?

I've also noticed that `bench_rev_1` and `bench_rev_2` are remarkably fragile. It looks like poking other code in `Vec` can cause the codegen of this benchmark to switch to a version that has almost exactly half its current throughput and I have absolutely no idea why.

Here's the fast version:
```asm
  0.69 │110:   movdqu -0x20(%rbx,%rdx,4),%xmm0
  1.76 │       movdqu -0x10(%rbx,%rdx,4),%xmm1
  0.71 │       pshufd $0x1b,%xmm1,%xmm1
  0.60 │       pshufd $0x1b,%xmm0,%xmm0
  3.68 │       movdqu %xmm1,-0x30(%rcx)
 14.36 │       movdqu %xmm0,-0x20(%rcx)
 13.88 │       movdqu -0x40(%rbx,%rdx,4),%xmm0
  6.64 │       movdqu -0x30(%rbx,%rdx,4),%xmm1
  0.76 │       pshufd $0x1b,%xmm1,%xmm1
  0.77 │       pshufd $0x1b,%xmm0,%xmm0
  1.87 │       movdqu %xmm1,-0x10(%rcx)
 13.01 │       movdqu %xmm0,(%rcx)
 38.81 │       add    $0x40,%rcx
  0.92 │       add    $0xfffffffffffffff0,%rdx
  1.22 │     ↑ jne    110
```
And the slow one:
```asm
  0.42 │9a880:   movdqa     %xmm2,%xmm1
  4.03 │9a884:   movq       -0x8(%rbx,%rsi,4),%xmm4
  8.49 │9a88a:   pshufd     $0xe1,%xmm4,%xmm4
  2.58 │9a88f:   movq       -0x10(%rbx,%rsi,4),%xmm5
  7.02 │9a895:   pshufd     $0xe1,%xmm5,%xmm5
  4.79 │9a89a:   punpcklqdq %xmm5,%xmm4
  5.77 │9a89e:   movdqu     %xmm4,-0x18(%rdx)
 15.74 │9a8a3:   movq       -0x18(%rbx,%rsi,4),%xmm4
  3.91 │9a8a9:   pshufd     $0xe1,%xmm4,%xmm4
  5.04 │9a8ae:   movq       -0x20(%rbx,%rsi,4),%xmm5
  5.29 │9a8b4:   pshufd     $0xe1,%xmm5,%xmm5
  4.60 │9a8b9:   punpcklqdq %xmm5,%xmm4
  9.81 │9a8bd:   movdqu     %xmm4,-0x8(%rdx)
 11.05 │9a8c2:   paddq      %xmm3,%xmm0
  0.86 │9a8c6:   paddq      %xmm3,%xmm2
  5.89 │9a8ca:   add        $0x20,%rdx
  0.12 │9a8ce:   add        $0xfffffffffffffff8,%rsi
  1.16 │9a8d2:   add        $0x2,%rdi
  2.96 │9a8d6: → jne        9a880 <<alloc::vec::Vec<T,A> as core::iter::traits::collect::Extend<&T>>::extend+0xd0>
```