[LoopVectorize] miscompiles counted loop at -O2 (off-by-one)

The following code is miscompiled in current trunk and all released versions since clang 19

``` c++
int32_t sumB = 0;
bool finished = false;
uint8_t counter = 0;
while (!finished) {
if (counter < a ) {
sumB += b;
counter++;
} else {
finished = true;
}
}
```
The bug is only there if a is 7, 15, 31, 63 or 127 and flag -O2 or -O3 is given.
It is not there with -mllvm -vectorize-loops=false

If Address or Undefined behavior sanitizer are enabled the bug does not show up

https://godbolt.org/z/rfjnPE8zd

Full reproducer code:
[clang-bug.cpp.txt](https://github.com/user-attachments/files/29336290/clang-bug.cpp.txt)

Start with the attached clang-bug.cpp.txt reproducer and the linked Compiler Explorer example. Reproduce the off-by-one result at -O2 and -O3 for a values 7, 15, 31, 63, and 127, then compare with -mllvm -vectorize-loops=false and sanitizer-enabled builds. Done means the counted loop produces the correct result without disabling loop vectorization.

Written by the indexing model from the issue text.