std: use ptr.offset where possible in the vec iterator.

Closes #8212.

Before:

```llvm
%sum.028 = phi i64 [ %23, %match_else ], [ 0, %"normal return" ]
%.sroa.0.027 = phi i64* [ %21, %match_else ], [ %12, %"normal return" ]
%19 = ptrtoint i64* %.sroa.0.027 to i64
%20 = add i64 %19, 8
%21 = inttoptr i64 %20 to i64*
%22 = load i64* %.sroa.0.027, align 8
%23 = add i64 %22, %sum.028
%24 = inttoptr i64 %20 to %"enum.std::libc::types::common::c95::c_void[#1]"*
%25 = icmp eq %"enum.std::libc::types::common::c95::c_void[#1]"* %24, %16
%26 = icmp eq i64 %20, 0
%or.cond = or i1 %25, %26
```

```
test vec::bench::iterator ... bench: 87 ns/iter (+/- 7)
test vec::bench::mut_iterator ... bench: 87 ns/iter (+/- 10)
```

After:

```llvm
%sum.028 = phi i64 [ %21, %match_else ], [ 0, %"normal return" ]
%.sroa.0.027 = phi i64* [ %19, %match_else ], [ %12, %"normal return" ]
%19 = getelementptr i64* %.sroa.0.027, i64 1
%20 = load i64* %.sroa.0.027, align 8
%21 = add i64 %20, %sum.028
%22 = bitcast i64* %19 to %"enum.std::libc::types::common::c95::c_void[#1]"*
%23 = icmp eq %"enum.std::libc::types::common::c95::c_void[#1]"* %22, %16
%24 = icmp eq i64* %19, null
%or.cond = or i1 %23, %24
```

```
test vec::bench::iterator ... bench: 79 ns/iter (+/- 8)
test vec::bench::mut_iterator ... bench: 78 ns/iter (+/- 2)
```

(NB. I'm not 100% sure the benchmarks were set-up correctly, so take them with a grain of salt.)