BPF: stack args cause backend to crash without optimization

When compiling the following code using `clang` with the BPF target, but without enabling optimizations, the backend crashes with the error `fatal error: error in backend: Unsupported instruction : >`.
```c
void f(int, int, int, int, int, int);

int main(void)
{
f(0, 0, 0, 0, 0, 0);
return 0;
}
```
A `store_stack_arg -8, r1` instruction is emitted for the sixth argument, but because the `BPFMIPreEmitPeephole` pass is only added when optimization is enabled, the pseudoinstruction is not replaced with actual stores. Running `llc -O0` on the equivalent IR also shows that `store_stack_arg` appears in the assembly output.

Reproduce the crash with the shown C program using clang and the BPF target, then compare it with llc -O0 on equivalent IR. Inspect the BPFMIPreEmitPeephole pass and the handling of store_stack_arg; done means unoptimized output no longer leaves the pseudoinstruction unresolved and the backend no longer crashes.

Written by the indexing model from the issue text.