Loop vectorizer generates inefficient code

https://godbolt.org/z/MPPnvT5h8

the simple code:
```cpp
void swap_ptr_impl(int64_t* ptr, size_t len) {
for (size_t i = 0; i < len; i++) {
ptr[i] = std::byteswap(ptr[i]);
}
}
void swap_ptr2_impl(int64_t* ptr, size_t len) {
auto end = ptr + len;
for (; ptr < end; ptr++) {
*ptr = std::byteswap(*ptr);
}
}

void swap_span_impl(std::span sp) {
for (auto& x : sp) {
x = std::byteswap(x);
}
}

void swap_span_2(std::span sp) {
for (auto& x : sp) {
x = std::byteswap(x);
}
}
```

swap_ptr_impl is 2x slower than other functions on i9-14900KF. 2.8x slower is seen on quickbench.
swap_span_2 (span length known) is also 2x slower.
```text
Run on (32 X 3187 MHz CPU s)
CPU Caches:
L1 Data 48 KiB (x16)
L1 Instruction 32 KiB (x16)
L2 Unified 2048 KiB (x16)
L3 Unified 36864 KiB (x1)
------------------------------------------------------
Benchmark Time CPU Iterations
------------------------------------------------------
swap_ptr 400 ns 390 ns 1723077
swap_ptr2 184 ns 180 ns 4072727
swap_span 176 ns 165 ns 4072727
swap_span_2 403 ns 399 ns 1723077
```

with -fno-vectorize, the results are reasonable.
```text
Run on (32 X 3187 MHz CPU s)
CPU Caches:
L1 Data 48 KiB (x16)
L1 Instruction 32 KiB (x16)
L2 Unified 2048 KiB (x16)
L3 Unified 36864 KiB (x1)
------------------------------------------------------
Benchmark Time CPU Iterations
------------------------------------------------------
swap_ptr 181 ns 184 ns 4072727
swap_ptr2 181 ns 180 ns 3733333
swap_span 173 ns 172 ns 3733333
swap_span_2 175 ns 173 ns 4072727
```

so I assume that there is something wrong in the loop vectorizer. Verified since clang 17.

Start with the Godbolt reproducer in the issue and compare the generated code and benchmark results with vectorization enabled and disabled. Trace the loop-vectorizer behavior for swap_ptr_impl and swap_span_2; done means the affected loops no longer generate the reported inefficient code without regressing the other examples.

Written by the indexing model from the issue text.