[IndVarSimplify] Allow predicateLoopExit on some loops with thread-local writes (#155901)

This is important to optimize patterns that frequently appear with
bounds checks:

```
for (int i = 0; i < N; ++i) {
  bar[i] = foo[i] + 123;
}
```

which gets roughly turned into

```
for (int i = 0; i < N; ++i) {
  if (i >= size of foo)
     ubsan.trap();
  if (i >= size of bar)
     ubsan.trap();
  bar[i] = foo[i] + 123;
}
```

Motivating example:
https://github.com/google/boringssl/blob/main/crypto/fipsmodule/hmac/hmac.cc.inc#L138

I hand-verified the assembly and confirmed that this optimization
removes the check in the loop.
This also allowed the loop to be vectorized.

Alive2: https://alive2.llvm.org/ce/z/3qMdLF

I did a `stage2-check-all` for both normal and
`-DBOOTSTRAP_CMAKE_C[XX]_FLAGS="-fsanitize=array-bounds
-fsanitize-trap=all"`.

I also ran some Google-internal tests with `fsanitize=array-bounds`.
Everything passes.

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