Repository: rust
Issue: compiler-builtins CI fails on powerpc64
Labels: T-compiler,O-PowerPC,C-bug,regression-untriaged

Some `memset` tests fail on both powerpc64-unknown-linux-gnu and powerpc64le-unknown-linux-gnu, but only with `--release`. Bisecting show #91743 as the culprit, but I think it just exposes an existing LLVM bug rather than being the root cause.

### powerpc64le

```
---- memset_backward_aligned stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 204, 204, 204, 204, 204, 204, 204, 204, 204, 0, 0, 0, 0, 204, 204, 204, 204, 28, 29, 30, 31]`,
 right: `[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 28, 29, 30, 31]`', testcrate/tests/mem.rs:265:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- memset_backward_misaligned_aligned_start stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[0, 1, 2, 3, 4, 5, 6, 7, 204, 204, 204, 204, 0, 0, 0, 0, 204, 204, 204, 204, 0, 0, 0, 0, 204, 25, 26, 27, 28, 29, 30, 31]`,
 right: `[0, 1, 2, 3, 4, 5, 6, 7, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 25, 26, 27, 28, 29, 30, 31]`', testcrate/tests/mem.rs:253:9

---- memset_backward_misaligned_nonaligned_start stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[0, 1, 2, 3, 4, 5, 204, 204, 204, 204, 204, 204, 0, 0, 0, 0, 204, 204, 204, 204, 204, 204, 204, 23, 24, 25, 26, 27, 28, 29, 30, 31]`,
 right: `[0, 1, 2, 3, 4, 5, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 23, 24, 25, 26, 27, 28, 29, 30, 31]`', testcrate/tests/mem.rs:241:9

```

### powerpc64
```
---- memset_backward_aligned stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 204, 204, 204, 204, 204, 0, 0, 0, 0, 204, 204, 204, 204, 204, 204, 204, 204, 28, 29, 30, 31]`,
 right: `[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 28, 29, 30, 31]`', testcrate/tests/mem.rs:265:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- memset_backward_misaligned_aligned_start stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[0, 1, 2, 3, 4, 5, 6, 7, 0, 0, 0, 0, 204, 204, 204, 204, 0, 0, 0, 0, 204, 204, 204, 204, 204, 25, 26, 27, 28, 29, 30, 31]`,
 right: `[0, 1, 2, 3, 4, 5, 6, 7, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 25, 26, 27, 28, 29, 30, 31]`', testcrate/tests/mem.rs:253:9

---- memset_backward_misaligned_nonaligned_start stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[0, 1, 2, 3, 4, 5, 204, 204, 0, 0, 0, 0, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 23, 24, 25, 26, 27, 28, 29, 30, 31]`,
 right: `[0, 1, 2, 3, 4, 5, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 23, 24, 25, 26, 27, 28, 29, 30, 31]`', testcrate/tests/mem.rs:241:9

```